Base of Run for Non Litter Trained Bun?

My bunny hasn't quite got the hang of going in his litter tray yet, so before I let him loose on the carpets I wanted to set up a run attached to his cage so he still has space to play in when I'm out of the house.

At the moment I let him out daily in a run which has cardboard on the bottom with puppy pads stuck on top. But I have to constantly watch him as he loves digging through these and I'm worried he'll eat the plastic backing :/

What would you guys recommend as a base for a more permanant run? Would lino or wood be okay if he's going to pee on it as well? Or should I stick a towel over these? It is safe to leave him on either without supervision?

How old is he? and is he neutered?

male buns seem to get the hang of litter training better after they are neutered - though there are exceptions.

If he is still too young for that try lino - though I would be 'kind' and put something over it - otherwise he will walk in liquid pee and spread it everywhere.