Well, amazingly, a cat managed to get past my half brick and the double twisty corridors. Like you, I was at a loss. They do seem to be incredibly persistent! and how is it physically possible? I added a complete brick as well as the half brick and that solved the problem. The hog didn’t seem to have a problem with it and neither did some of the returners. But having seen the previous attempt I was worried in the mornings whether I was going to open the box to find a cat stuck in the twisty turns. Thank goodness, not.
Only one of the 6 cats who frequently visit the garden have visited in the last few weeks. Ironically just when a rat turns up. Are all these cats complete wimps!?
Seriously, though, I think it is harder for the cats to twist if the area is shallower. They don’t seem to be able to get through a shallower longer bit with a turn at the end. I’m not sure whether you are going to be able to do it with your current set up, unless you can create a long shallow tunnel with turns in it, outside. You might end up with the whole garden full of a maze!
If you are thinking of starting again, Penny tells us that her box seems to work and she used to have problems with cats. There is a picture on the following https://www.flickr.com/photos/144453064@N05/35319840883/
She did put the measurements somewhere, but I can’t find that at the moment. If I come across it I’ll copy it here. Or maybe Penny might see this and tell us again. If you do use that as a copy, I think the key is to keep the entrance holes quite small, because otherwise the entrances set up is very similar to mine and as I said, the cats got in.