toilet in corners

[lorrie]

Hi there, I have just got a couple of sister rabbits, lopeared lionheads, and at the bottom section of there double cage I put down paper & sawdust. But they decided to move all of it away from the corners of the cage, and wee there, and it's on the wood of the cage, which will be no good for it. I then tried sawdust and straw, and that didn't stop them, also got a corner litter tray, which I hadn't fixed in there cage yet, but they ended up pulling this out to the middle of the cage, and decided playing in it was a better idea.
Does anybody have any ideas as to what i should use on the floor of there cage to stop there wee soaking through?
Lorraine.