I'm hoping someone else who knows more than I will come up with a better
answer but here's what I know. The bas relief type of molds, which I think
is what you are talking of, is made out of vacuum formed plastic. To make
it a solid model of the object is placed on a holed plate (something like an
air hockey table) a heated sheet of plastic is placed over it and sucked
down onto the object. If you could make some of the object out of metal or
some other heat resistant material you could then take them to a vacuum
molding company which could make the molds for you. Not every company would
know what type and thickness of plastic is needed for chocolate molds so you
may have to do some research there or give them a sample. Some airplane
modelers also set up home vacuum molding rigs to make parts for their
models.

Charles P. Lamb