Hi. We've had some posts on TackleMaking.com about making soft plastics since I originally posted this here. Here are some links to "how to" and design tips. There are also some images of how to make molds in the forums section:
Here is post by "Pole Pimp" (a member) that shows the process for a homemade soft plastic mold. The top and bottom halfs are made by pouring plaster into a metal pie tin. Pins are added to the plaster to keep the mold straight.