you can also buy plastic litter trays designed for corners, they're only a few quid, and my ferrets use them.
the pets corner bit at Weston Longville Dinosaur Park (giant fibreglass dinosaurs in a wood, shit for adults, but kids love it ) has a big ferret court. i was there with my mates autistic nephew a couple of years ago, he wanted to see the ferrets, so they let me get a couple of them out. i was chatting to the girl who looks after them, and she said there was 7 ferrets in the pen. i could only see 6, so i asked about the other one. "oh, he's up there!". she pointed to a 6" wide le