Liquid plastic casting resins tend to be expensive and have set properties that limit your flexibility. Mikey77 has come up with his own material out of materials so common, chances are you already have them. Best of all, you choose the color, viscosity, and set time yourself based on your needs.
He calls the stuff Oogoo, a name reminiscent of Sugru, an off the shelf product with very similar properties. In a nutshell, his recipe is very simple, just mix corn starch with silicone caulk.
The problem with silicone is that it takes a long time to dry from the outside in, the corn starch allows it to dry from the inside out, and very quickly. The ingredients are pretty safe and stable, so you can mix in your own dyes and other ingredients. Check out his full instructable here.