Thanks for the advice. I think I'll just build the coroplast so that the cage basically sits on top of it. My oldest bunny was litter trained about 6 months ago before he was moved to a cage with the wire bottom so I hope that he can re-adjust again to a plastic bottom. I had him moved because that cage was too shallow for him and plus I got a new bunny that he bonded with. This new cage will be large enough for both of them, so hopefully they both can be successfully litter trained. Any advice?
Also, the older of the two is neutered while the younger is not. Is it possible for two rabbits to stay bonded even though the younger is not neutered? (The younger is 7 months old, the older is 2 years old. Both are mini flops)