Try tiles. They are super easy to clean but some rabbits don’t like the slickness of them so put a towel or blanket down so they have a little traction. You could try sticking them to cardboard or a safe sheet of wood. I did the cardboard for my little guy, and it worked well. I made sure that there were not any/many cracks between each tile so if he peed it would seep through. Luckily, he has really awesome litter habits and never peed on it. The only thing that did happen was he tipped over his water dish a couple times. Cardboard is more for a temporary solution. He’s on a dog tray now, which would also work for you. I got mine off of Amazon. I don’t remember the measurements, but its a good size. Actually, it would fit the cage you want to build. I used it for my other 2 bunnies and my cage was 3 wide, 3 high and 2 deep. There was a little extra tray on the sides but it still worked really great. It’s not as easy to clean as tile but I thought it worked well.