If you do try to make them in your mold, there are certain types of plastics/polymers that can be melted. Basically you have thermoplastic polymers and thermosetting polymers. Thermoplastic can be melted and reformed, but don't use a direct flame or they might catch fire. A simple one to use depending on what it's used for is Polycaprolactone (google it) because you can essentially melt it in a hot water bath. Anyway, this is all just fyi. Good luck!
P.S. Polycaprolactone is sold as polymorph, shape lock, and a couple other brand names.