1/16" or 3/32" sheet lexan epoxied on the foam and vacuum bagged will make an excellent surface off which to pull a mold. Will need to have the foam undercut the correct amount to accommodate the lexan thickness. Material is availble at McMaster Carr or you can get something similar at Cheapo Depo

The process of building the mold is fairly simple actually. A sheet of lexan could be used a a parting plane in the center of the wing. A high quality wax is a must of course to release without any fuss.

All of my molds start with a layer of epoxy, carbon black powder and colloidal silica for smooth surface. Thickness is around 1/16". Then the reinforcing layers are applied. The black coloration provides contrast and allows me to easily see where the ensuing glass or kevlar layup may need work. Carbon cloth is a more difficult to see such flaws but in general, carbon cloth works well and isn't an issue

MattK