I have the one that horza posted a link to. I think it's probably the biggest I've seen.
Other than that, I think Jo's idea of a storage box is a good one. Many people on a US forum I occasionally frequent seem to do that. You could get a high sided one and either cut it down at the front for easier access (although I'm sure Leo would have no trouble climbing in the top!) or cut a hole out in the front for access.