If you do get a wood one, it's a good idea to paint it inside with plasti-kote water based enamel. It's petsafe and virtually odourless and dries quickly. A couple of coats makes it wipe clean inside in case it gets pee'd in or soiled
The Plasti-kote costs about £4 or £5 but a little bottle goes a very long way (in case you wanted to use it on the shelf as well!)
A cardboard box house is disposable and easier to replace although it's surprising how you can never find a shoe box when you need one.
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
It doesn't have to be an add-on - I get it from one of the other sellers (A P Motors) or hardware shops usually sell it. But it needs to be the water based paint on one really.