For small batches of plastic, a 100 ml beaker works great. Get 3 of them and heat all 3 colors at one time, then pour. Plastic won't stay hot long, but with just a few molds it should be ok. You can get the beakers from a scientific supply or if you know a compounding pharmacist, he can order you some pyrex beakers. Make sure they are PYREX.
I wouldn't suggest heating the pyrex or anchor hocking cups on a hot plate or griddle. JMHO. I have heard people say they crack that way. I only use microwave to heat plastic.
The metal pans are a pain in the side. Once you start using the microwave, you will be happy with the results.