I use the standard Frabill minnow traps in one of my favorite bullhead spots. I get bullheads but they are the smaller ones: 4″ – 5″ bullheads. The traps work good, a friend of mine shared a technique he uses to improve his minnow trapping results. He recommends placing the traps so the openings are parallel with the shore. He felt he caught more minnows that way – so that is how I always try to place mine when I put them out. I think what happens is the trap catches minnows and the bullheads go in the trap after the minnows. When you take the bullheads out of the trap they are just fat and full of minnows. I usually put the traps out in the evening and check them early the next morning. If there is nothing in them I will move them until I find a spot. Once you catch some in a particular spot it seems to work pretty consistently.
The regulations on traps in listed on page 70 of the 2006 MN Fishing Regulations under Minnows and Leeches. I was going to make the opening bigger on my traps to see if I could get some bigger bullheads but the regulations specifically states the opening may not exceed 1 1/2 inches.
The reg also states that the traps must have a waterproof tag bearing the name and address of the owner. I made some business sized cards with my name and address and laminated them and just used a cable tie to attach the laminated address card to the trap.