If you need a small part and you have access to the original then use some JB weld to get 'er done.
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
Take some kind of casting medium, I stole some playdough from my son. I think this would work even better if you used modeling clay or something made specifically for such an application. Who knows.