You beat me to it, that is the video, ( but not the same as the David Gardiner vid)
I personally would not use Al for the panel (except as a template, but not that big ?..I use .010" roasting pans ) …a paper template should be ok
It is possible to use rivets and bonding with gal (imo).
I have a mate that does all his repairs this way, and he really knows his stuff.
I have done them, and they last as well or better than welding in some applications (stood the test of time)
depends if the floor pan flange is intact (you would be lucky)
welding isnt easy in this area