This is the trap I made to catch my mantis shrimp. He wouldn't go for it on the first day, but he ran in and got stuck on the 2nd night, so you might want to give it a try. I got this from someone else's posting on another forum. Good luck to you.
The one I made is as follows--apologies to the inventor, I hope he'll post and claim it!
1. Get a piece of 1" dia PVC, 12" or so long. Leave a bit of a burr on the inside edges on one end. Any other tube will probably also do.
2. Get a plastic soda bottle. Cut a rectangular piece out of it's side. The dimensions should be 3.25" (parallel to the bottle's axis) x 1.5" (along the bottle's circumference). The plastic rectangle will be curved; the long sides will be straight, the short sides curved.
3. Make a series of cuts starting on the straight side. The cuts should be about 1" deep and about 1/16" apart. What you will end up with is something that looks like a 3" long hair comb with 1" long teeth.
4. Cut every other tooth off.
5. Cut every other tooth that is left to about half it's length. Of every 4 teeth you started with, you now have one full length tooth and one half length tooth left.
6. Bend the spine of the comb into a circle so that all the teeth curve toward the middle. Insert this, teeth first, into the end of the PVC pipe. This makes a one-way door--the mantis can push his way through the teeth to get in, but he can't get out. The burr you left on the inside edge of the PVC will prevent the comb from being pushed back out the end of the pipe. You may want to do some further trimming on the comb to get an exact fit.
7. Cap the other end of the pipe. I jammed a left over bulkhead strainer into the end like a cork--easy to remove but allows water to flow.
8. Put a piece of bait (raw shrimp, clam, whatever) in the trap. It is probably best to orient the trap with the comb end on the downstream side, as you want the mantis to follow the scent into this end.
9. Check it daily. The bait will stink after a day, so change it daily. I think it also helps to chum the water a bit, get the mantis used to eating the bait.