Those minnow/crayfish traps are very good at catching snakes. For cottonmouths you need a hole big enough for them. You can get a nice crayfish trap with an extension that makes it about 31 inches long or so for about $15. You might have to make the holes a little bigger than the 2 1/4" size that it comes with.
I have caught black racers accidently by leaving minnow traps in my yard.
