I use the foam you mix part A/ Part B. I have used it for many things. Here is a repair I used it for after a belly landing. Made the stringers but used it to fill in as suport:

http://www.rc-tech.net/bearcat/belly/b1.jpg
http://www.rc-tech.net/bearcat/belly/b2.jpg
http://www.rc-tech.net/bearcat/belly/b3.jpg

I have also found it as a great glue inside fiberglass airplanes. I have glued in gas tanks and other things like that. I tried making some molds and realized I could not get the foam to release. That is when I discovered it sticks to nearly everything. It fill as it does. This is where they came up with a modified version of it as "Gorilla Glue". I buy it from Fiberlay:

http://www.fiberlay.com/