Well, it's not a trap but, during open water season I usually take my casting net down to the stream that discharges from the Lake I'm going to fish. I can usually catch a few dozen pretty quick. Just check your states reg's. If they allow casting nets, most usually have limits on the size of the net and size of the holes in the mesh. During Ice fishing season the stream may have open water close to the dam depending on how the weather's been and if it's a spillway or some kind of regulated discharge.
I havn't tried this yet but I've heard of people making their own bait tanks from those cheap 20 gal. resin/plastic laundry tubs (the kind with the legs and holes for plumbing, home depot sells them for like $20) and an air stone. Just have to change the water every couple days.