JB weld did the trick on a missing piece for my airsoft gun.
UPDATE - Make sure you check out all the awesome comments about how to go even further and reinforce the epoxy.
Step 1: Playing with Playdough
Step 2: Your Part
One trick I did learn was that for the really deep parts that it helped to fill them with the clay so that it would then adhere to the rest of the mold when you remove.
Also, try to make sure the top of your mold is even with the top of your piece to avoid too much excess.