Cardboard no good unfortunately, the reason I need the lift off roof is because Harvey has taken to weeing inside the house instead of the potty - so cardboard would get pretty soggy, pretty quickly!
I love the look of the rodipet ones, especially the two rooms - as then I could try to set one up for weeing and the other for sleeping. In her old cage, the nest and potty were under a shelf, and now the potty is in the open and I think she preferred the privacy (or she could just be getting lazy!).
Thanks Coco for the picture suggestions, will go and take a look