I haven't tried and I don't think I could be bothered (that sounds really lazy i know), but sometimes you see them cheap here in $2 shops (if you have those where you are) I picked one up for A$12 once and the last one from the pet supplies place was only A$20.

They get terribly messy once they "go" don't they?

Have you thought of putting some rough/heavy duty carpet round it? That would be a lot easier. I'm sure one of the carpet shops would sell you an offcut. Or several wrap-arounds of hessian/sacking (clean of course lol)