I dig it Sofa!
I am planning on using your foam idea to smooth out the buck I made for my front end. I just hope my 'glass work turns out as good as yours did!
If you're talking about spray foam in cans, be aware the it doesn't sand easy like the foam blocks/ sheets. It has a more rubbery consistency. I found it easiest to cut it to shape with a razor knife. I use the ones that have the snap-off sections. You get a 4" blade that way. With a little practice you'll get pretty darn good at it. Also, don't try to fill large areas with it. The outside will cure leaving the insides an uncured and gooey mess. The stuff cures with moisture so I spritz it with water to make it cure faster. When I don't finish off a can, I lay the can on it's side and shoot out a small ball of the stuff and leave the tube and can stuck in it undisturbed. After that ball cures you can store it for weeks, pull off the ball, pull off the tube and run a coat hanger thru it to push out any cured stuff and you're good to go. The stuff is cheap but gets expensive if you just use a little and then throw the rest away. The canned foam works really good for locking the foam pieces together.
Also, I found a spray contact glue that doesn't eat any foam - Elmers Fast Tack. Bonds in 15 seconds, too.
Be sure to cover your buck completely so no resin gets to (and eats!) the foam. I use that aluminum tape because it seals really well, is super sticky, holds it's own shape and will lay down super flat if you rub it with something (like the side of a magic marker). When they built the Buddfab streamliner body they covered their foam with 3 coats of gloss latex house paint, release waxed it and laid their glass right over it!
I don't have the nerve to try it.
Good luck.