We make parts, shape, and add form to existing plastic parts using a chemical called "molding epoxy." You'll find it at most hardware stores. They use it to fix leaky pipes I think! It's a 2 chemical compound you mix them together and it "sets" (hardens) in a certain amount of time. I recommend getting the kind that sets in 30 minutes or more. That way you have time to sculpt it exactly the way you want. It has the consistancy of clay before it sets. After it hardens it's permanantly as solid as a rock. Works awesome for fixing toys or "making parts." For instance, I sculpted Transmutate's head using it and added it in certain places to make cheetor's beast face resemble Ravage's. Here's a picture of the stuff we've used, but there are many different brands all of which work well.

Make sure you have a well ventilated room to work in. The stuff stinks real bad. No it's not a pleasant stink. If you don't have a nice window to work by get one!