Yesterday I stopped by MushroomBoys to pick up some frags. He had a trap in his tank because hes taking down his reef, and he said it worked great. I think the most important thing is, don't expect to have it work instantly, they have to get comfortable with the trap being in the tank a couple of days.
I just use a clear two liter plastic pop bottle. I cut and invert the small end inside the larger part ot bottle. The fish will go toward the bait and be funnelled into trap and not get out. I used popcorn shrimp on a piece of thread for bait. That is how I caught my problem wrasse. I hope this helps.