Linweir.. if it's not accessible from the inside (ie foam showing) then you can't weld it (or you'll burn the boat up) and you can't solid rivet a patch either. I should have the required blind "tank" rivets though to make a leakproof seal and I do have the aluminum for sure.
I'll send you my email in a PM and you can get your buddy to email me from there with some pictures.