Several years ago, I read of someone making a mould with Portland cement slurry. Apparently worked well, but would likely need a good release agent. Two half boxes, make one side (with support for the wax foundation used as the initial mould) and when cured simply turn over and pour the other side into the lid.
Fill mould with wax and close down other half the box it was made in. I thought it may trap air on the top side, but not sure that would be any particular problem anyway - as long as the sheet did not come out perforated too much!
Silcone seems a much better, if more expensive, media to use.
Regards, RAB