I haven't tried minnow traps or the like, but I've had reasonable luck in some places catching madtoms with a dipnet.
* work the banks of small creeks, especially vegetation, root balls and underhangs
* in slow areas, sometimes they're down in the leaf litter in slack backwaters
* if there's sunken logs or branches, work those with your feet while holding your net down in the likely direction of escape
* some species are out under the rocks in reasonably fast-moving water ... do a darter shuffle for those and kick the bigger rocks
* my most productive and consistent spot for tadpole madtoms is in a 70 yd wide but shallow spring run here in Florida, out in the middle where the water is 3-4 ft deep, down at the bottom in deep & thick valisneria (eelgrass) beds. (edit: working that stuff is probably part of the reason why I have a collection of 7 broken dipnets in my garage.)