So, I was actually studying (!) and I thought of something.
Do you think that Great Stuff would absorb through fiberglass mat? Obviously not, if you put resin on it and allowed it to cure, but I was trying to think of something better than my foil-backed foam, something workable.
My other thought was, have you ever opened a package and there was a protective sheet of really-thin foam? What if I took a sheet of that, lined my fog light socket with it, and then sprayed foam? The two types of foam would bond and then I could work that, if I needed to.
Maybe this?
Staples Anti-Static Air Foam Dispenser Packs, 175' | StaplesŪ
This place has them cheaper, although I do not know about shipping and handling:
http://trisupplies.shoppkg.com/shop/C2285.aspx
Those sheets are 12 or 24" across.
What do you think?