Tipping litter trays

My rabbits continually 'play with' their litter tray. They move it about and tip it up. It is driving me nuts!!

Anyone got any suggestions - apart from casting the tray in concrete!
:lol: :lol:

The woman I get my rescue buns from advises you to put a couple of roof slates in the bottom to stop them tipping it up (but not many people have roof-slates to hand do they!!? :lol: ). I know other people on this forum have recommended putting a brick in the tray.