Some of our locomotives had a 4"X24X24" air filter for the cab, the sides were the 4" sheet metal. The sheet metal covered the tops about 1 inch. I dismantled a few to make 1" corners. The filters had either 1/4" expanded aluminum or plastic about the same to hold the filtering material in place. I made a few traps with these. Expanded aluminum or plastic on the sides instead of sheet metal. Some were 4 through 24", 4X24X24 to 24X24X24" We catch silver sides and crayfish mostly.