I saw a site that showed a crab trap made out of a plastic pop bottle. You have to cut the top portion off and reverse it and put it back into the bottom portion. The crab can get in, but can't get out because of the angle. You can use a hole punch and some fishing line to secure the two pieces together if they don't fit snug enough. You may also have to widen the opening - or use one of those wide mouth, one liter bottles, depending on the size of the crab....
Hope that works for you.