borrow someones puffer fish.
you can easily make simple traps useing things from the craft store. a clear plastic box with lid, rig it so that you can pull it up right when the crab goes in. have it on its side with bait in it, string tied or glued to the edge and when he is inside, pull it up. if you can make some kind of door that can be triggered to close with a pull of a string then it should work better.
i have one now that im simply growing and will soon let him finish growing in my sump. i got him dirt cheap and the local fish store gives some good credit for huge emeralds