During a light sprinkle rain yesterday I caught several 4-5" bullheads by hook and line. Everything was biting for about 30 minutes. So it looks like I may have caught some of the larger ones in the trap, but the smaller ones probably are too small and find their way back out. I may try to reduce the opening size of the trap because I think the mesh size is small enough to keep them in.
I bought my trap at Acadamy Sports. They had a couple different styles. The one I have is rectangular with openings in each end for the fish to enter and a bait container in the center. I can't tell you if it is a good trap or not, because other than a minnow trap it is the only fish trap I have ever used. I've caught several fish in it though.