I don't know how fiberglass mold release spray would work for you, but I used it when making fish tanks, and it's not that expensive. I bought it in quart cans and sprayed it on with a PreVal sprayer kit.
Fiberglass mold release was what I ended up using after I moved away from using the silicone mold release agent (expensive that one is) but the tried and true way worked even better than the fiberglass release agent. it just take 5-10 times longer. I'm probably going to go to a modified setup that let's me apply one coat of carnuba wax, let it dry and then spray (harbor freight gun just for mold release.)the heck out of it with FG release. it'll save time I hope.
