I have a great idea on how to make a foundation mold and its will be very easy and re usable. You'll just have to provide wax after making the mold!
Go to Hobby Lobby or some sort of hobby place that has lots of stuff.
Get this latex stuff that you make molds with.(It looks like a little tub of elmers glue sort of).
Get a piece of plastic foundation.
Paint a few(Ok, several layers, letting it dry between coats) of this latex stuff onto the foundation. Actually, you will need 2 pcs of plastic foundation so you can make a mold of both sides.
After it dries, peel the latex off. You now have a floppy pc of latex rubber, right?
Dont forget, you need to do this for front and back.
Now lay the latex rubber down on a flat surface with the cells facing up( a small pc of plywood will do) and pour hot wax onto it, tilting it around to keep your wax from becoming too un even..Now do the other mold the same way. I think, if you get the wax just the right thickness, it will come out of the molds in one pc. Also, maybe chilling the mold and wax will keep the wax foundation part hard to make the extraction easier Now put the wax pcs back to back and trim them to the right size.....Voila!!! You now have a pc of foundation you made yourself!..Stick it in the frame and see if it fits!
Hmmm,....After all this instruction, I think you ought to just buy the stuff. It seems it would take a long time to make enough to just fill one hive box. :-\