The diff cover is bolted to arguably the strongest casting on the car an the ribs only stiffen the casting against rear impact.
The alloy section is actually far thicker than the steel pressing and not significantly lighter.
By the time you have been rear-ended hard enough to need to reinforce a diff cover against the impact of bodywork further astern, a dinged stock cover would be the least of your worries if you survived.
As it happens, I do have a reinforced alloy diff cover from a Reliant Scimitar.
The casting includes a mounting at the apex of the dome for a Watts linkage.
I have fitted it to my D-Type instead of an A arm.
Well went to fit the cover today , there was only a few mm of free room , too close to the tank for me , so put the old one back one .
Alloy one will be up for sale in the new year 
A few millimeters is all ya need!
Shame, especially after the lovely hand made gasket. Thats gonna contact I reckon.
When I bought my Mk2, diff cover (standard) hit tank exactly in same spot as in Ian’s photo. Not all the time but in bigger humps with passengers on rear seats. As there is no way to adjust diff position (and all bushes replaced), I had to file tank fastening holes bigger and tank edge smaller so I was able to move tank slightly back, it was a question of millimeters. I wonder if my car has had a hit to rear and tank pushed forward or why was diff contacting tank.
Some photos of that in http://www.elisanet.fi/kopik/jag/restor08.htm
Köpi
I did think about looking more into it to fit the Alloy cover , I thought at first it was just a case of taking some alloy away at the odd fin , but no .
As no one would no it’s there , and it would make no diffrence if it was fitted or not , I decided , to just replace the old one , with new bolts , spring washers , and of course the new gasket I had made , as there was a slight oil leak , and I wanted to change the oil , it was not a complete waste of time .
Plus I had another issue to look at 
see new topic !
