Fleece or carpet, held down with cable ties works a treat, carpet is much easier to keep clean, though...It just needs replaced every few months
It's easy enough to get the carpet from carpet shops though, like the out of date stock thingies, the samples...A place near me sells them 3 for £1, I'm sure most places would help you out though
I wasn't saying it to be like a personal attack or anything, it's just that I saw a poor rat with bumblefoot today and it was awful