I will also chime in with the 3m Super 77. It will eat the foam but not much....
I have found the best method is to spray a light coating on both pieces to be joined, let sit for about 3-5 minuets (adjust time depending on temp, just to tack up a bit and flash most the solvents off) and carefully assemble.
Once the form is made and smooth (*drywall mud works well if you need to add material providing its not too deep) coat the mold with aluminum foil (*again spray with 3m 77) and roll the foil smooth with a laminate roller.
THe foil will act as a barrier to prevent release agents, chemicals, epoxy or other from attacking the mold.
After that you can glass or carbon fiber away.
Good luck.
~JH