Don't know if this is of any use for anyone with less able rabbits, but thought I would put it up here:
https://www.zooplus.co.uk/shop/cats.../1382910?origin=hopps&q=litter tray&i=46&ro=1
https://www.zooplus.co.uk/shop/cats.../1382910?origin=hopps&q=litter tray&i=46&ro=1