Hi GeordieBloke
If you have a cat problem, I’ve found 5 inches is too big – I had mine reduced to more like 4 inches, but some cats can still get in. I have found the best/only way, so far, of stopping persistent cats getting (if there is already a corridor with turns i.e. mine have double back turns) is to reduce the height of the tunnel/corridors by putting a couple of bricks, one each side of the turn.
The theory was that if the cats are having to flatten to get through the reduced height they would find it more difficult to double back on themselves. The hogs being shorter and able to squeeze through quite small spaces manage fine.
My boxes turn immediately left from the entrance, so a heavy object to the right of the entrance can also help – the cats normally try to enter at an angle but can’t do that with an obstacle there. If there is a double, corridor that works best. i.e. enter, turn immediately left, followed by a double back turn right along a bit before turning left into the feeding chamber with one brick in each corridor part.
If the corridor is straight into the box, it may be more difficult, but you could still try reducing the height. But it’s easier for a cat to go straight. However, you could try putting a heavy object opposite the entrance so that it can only enter from an angle – which would be more difficult if the tunnel is straight from the entrance. (or place the tunnel facing a buildng)
If it is one of those with a long tunnel leading from it, you could reduce only the actual entrance as opposed to the whole tunnel. Even large hogs can usually squeeze through a shorter smaller area. But large hogs can usually manage a brick length as well – although in a long straight corridor it’s harder to fix only one in place.
Hope all that makes sense. Not so easy when you can’t see it! But you will probably find it is a bit of trial and error to get it just right so that the hogs have no problem but the cats can’t get in.
Good luck. Hope you find something that works.