I bought a 4" lizard RTV mold from Lurecraft, and had trouble pouring it, due to the small legs and thin tail.
I figured out that if I poured the plastic, and then pressed a smooth faced piece of melamine down onto the open mold while the plastic was still hot, I got thin arms and tails. I do have to trim the entire lizard, once it's cooled, which is a pain.
The plastic sticks to the melamine, so when I remove the mold, I use an exacto knife to trace around the entire lizard, which is still stuck to the melamine, to trim it. But I have to be careful, because it's really easy to amputate a leg.
I now use NorPor silicone cups, instead of Pyrex cups, to melt and pour my plastics, and it's much easier to pour thin stuff. I wear gloves, due to the heat, and pinch the top of the cup into a small opening, so I can get a finer line.
It works better for me.