After extensive conversations with Mike Balch, we have spent a considerable amount of time developing a Leather Trunk Strap to hold the Tool Bag / Roll in place in the luggage compartment. A Center Bridge Buckle similar to that used for the Tool Roll has also been developed and completed. It takes a 5/8" Brown Leather Belt with indented grooves pressed into the top surface of the leather along both edges. Pictures will be posted shortly,
Looks very, very close. I was trying to find a photo of the original, but cannot locate it.
Here is the location it should be mounted at:
Tadek
and the question is
screwed on
bolted on
riveted on?
Rivet, aluminum Sir! 
I went stainless though, but do not tell anyone.
I believe early cars had a screw there, and there was a Service Bulletin #88 about it, how it was too long and there was danger of cutting into the tire on hard cornering, so the advice was to put in a round head screw and nut going the other direction.
A pop rivet would make even more sense on the production line, being faster to install.
rob is totally correct as usual on this early cars had the long self tapper!
and yes Tadek’s pic shows the remains of alloy rivet
For years, I’ve been frustrated that no one seems to make an accurate copy of this leather strap. I had collected photos, and all the measurements, of the leather strap and buckle from original survivors but I was never able to find anyone willing reproduce it.
I contacted Karl Robertson, who enjoys making very accurate XK120 tool rolls, (as well as Ferrari luggage) and he said it would be no problem.
Being a perfectionist, Karl wanted to get the buckle correct. When making the XK120 tool rolls, Karl found that no one today makes an “accurate enough” copy of the 1/2 inch wide center bar buckle originally supplied on factory tool rolls. So Karl had a small run of the tool roll buckles custom made.
The same is true for the leather straps that hold the tool rolls to the left side of the XK120 boots. The originals had a 5/8 inch wide center bar buckle, and no accurate examples seem to be available today. Thus, Karl went back to his “buckle guy” who made another small run of the 5/8 inch buckles for the leather straps.
Karl is retired and does this type of work as a hobby, not a business. He truly enjoys pursuing the small details. I’ll post a photo of my strap after I install it in my XK120.
Seems like interest in these Tool Kit straps is on the boil again, after last time it was raised, but certainly knowledge has improved…
Where I am at now is that over the 1949 to 1954 XK120 period there was at least two different BD.3437 Straps, an earlier black strap and a later brown strap similar to those being made/pictured above, albeit I would like some details of dimensions before commenting. But seems both straps share the same design/style Buckle - although my resercah is embryonic so cannot be conclusive (yet).
But the issue of how it is secured to the inner panel has been raised, and as noted this is discussed in Service Bulletin 88 (of January 1951) which suggests that from about January 1951 the original/previous use of a self-tapping screw (and cup washer based on original sightings) was superseded on the factory floor by the later use of an aluminium pop-rivet (dimensions and type of screws/washers and rivets have been noted), but also presumably maybe a few in-service modifications as per SB.88. So although extremely embryonic, one suggestion with no evidence base at all, is that maybe the black straps were superseded by the brown straps at the same time that rivets replaced self-tapping screws ???..
Now the other killer question is, exactly where was the self-tapping screw located, and indeed where was the later hole drilled for the pop-rivet…
What I tell people is have a look at your own car for a hole or remnants of a screw or rivet, and re-use the same position/hole, as although minimal data, it would seem that there was no template used to provide a consistent location for the screw/pop-rivet, suggesting more like just being done by hand one by one in roughly the same area - as so far originals rivets have been seen in variable locations…
So would love more feedback of anyone with what they think is an original strap, and advice whether Black or brown (other dimensions vary as well, but the colour is obvious), and indeed their Chassis Number/date, especially cars built in 1950 and 1951, which at this stage is best guess - albeit any comment from 1952 to 1954 owners also welcome…
But with Karl now making replica Brown straps with what looks like extremely accurate Buckles, we at least now have a second source/supplier of authentic brown straps/buckles - and also one supplying the earlier black straps/buckles… (again as with my earlier comments on authentic XK120 tool rolls/straps/buckles contact me direct if you want contact details - the maker/supplier does not advertise)…
But it would be great if we could get a better feel for black to brown strap demarcation…, rather than just at a guess, somewhere ‘maybe’ / ‘coincidentally’ same time as screw to rivet change, around January 1951…, but may well be many months different… tba
Next time I update my published Original XK120 Tool Kit paper, I will further expand on current detail on Tool Rolls, but will also be fully detailing this BD3437 Strap and fasteners, in same logic as also now detailing all the clips/straps that secure the various tools not within the tool roll, given the interest in such detail is certainly there…
I would like to add to this discussion about the XK120 Trunk Strap. I put up images of a Brown Strap because it was the first one made at a request from Mike Balch. As you can imagine, the Buckle has taken a considerable amount of time and effort to realize. At this point in time there are only two in existence so they were made into 2 Trunk Straps. The Black Belt, BD 3437 variant will be available within the week when the next Buckles arrive. In short both variants should be here soon! Watch this space!!
Karl Robertson
Attached is a photo of that strap in my car -1953 DHC S/N S677159. Build Date - June 1953. The car itself is quite original and I would have every reason to believe that this is an original strap. It is pop riveted about 7" behind the rear bulkhead. Can take more pictures if desired.
(The plywood, etc, above the strap is the endpoint of a shoulder harness that I put in as part of a 3 point seatbelt. The seatbelt is obviously not original to the car, but I thought it would be a good idea, given that it does not have a collapsable steering column either.)
.
And here are some replacement Trunk compartment Belts, BD 3437, shown is one in Brown. It is 5/8" and has the correct Buckle for that size width.
THats great Thomas,
Brown strap and chrome-plated buckle are as expected for August 1953, but yet another variation re position of securing pop-rivet, reinforcing belief that the strap/pop-rivet hole was just drilled by hand in approximate location, by assembly line worker, as/where he saw fit - hate to get this job done on a Friday afternoon 
Still looking for strap pics from 1950/51 XK120s…, to try and narrow down black belt to brown, and screw to pop-rivet demarcation…
Rather than a self-tapping screw, you could use a riv-nut with a slot headed machine screw. It would look original and there would be no nasty sharp screw point on the other side of the panel. I hate self-tappers!
My small contribution to this discussion is that the hole in the inner fender of my Nov '51 car is .116" diameter, so too small for a pop rivet.
Karl,
Just wondered if you are making some of these to sell and if so, can I purchase one? Even though my car still has the original one, it is pretty frail and will likely break at some point. Would like to have a good spare.
Thanks, Cliff
Hello Clifford,
The Trunk Straps, both Black and Brown have been developed for the small group of XK owners, so yes, they are for sale. Please contact me for further details. Thank you.
Karl Robertson
robertsonpdt@comcast.net
+1 248 766 9141