This is a very simple and fast mold for casting plaster or epoxy. I'm sure the picture explains it all, but to make things all good and proper, I'll go through it step by step.
NOTE: This isn't my idea, I'm just reposting it, because I think there are many more applications for this technique. The idea comes from Hirst Arts Fantasy Architecture. If you are interested in this project, I suggest you go take a look there at www.hirstarts.com
Step 1: Push in original
Push the original object you want to cast into the clay and pull it out.
Step 2: Pour
Pour the plaster or epoxy into the mold. I used Durham's Rock Hard.
Step 3: Let it dry
Let it dry and peel it out. Bam!
This technique isn't super precise, so you might need to do a little clean-up.