Been making the tunnels 600mm long 200mm high and the same 200mm wide across the top, which in the 12mm marine ply used made them around 180mm internal width. End sections were 20mm ply with tunnel shape cut-out. Also have some mk 6 traps for the rabbits that are eating the new coppice growth which the above box just fit over.
Not personally had any unwanted species trapped but will still persue jillybean's idea of a suspended the trap to lessen the risk. Peanuts and dried maize work for me as bait with a dab of peanut butter (as it has a much stronger smell to my human nose so hopefully attracts over a wider distance).
Caught 7 this year in the same area but bark stripping continues.