Possibly take out the wooden floor of the bottom part so that you can put a sheet of vinyl/fleece under there and have the edges of it reach up until it’s a bit outside of the cage, so that your bun can’t get to the edges to nibble on it. The fleece you could then wash every day if he pees on it, the vinyl you could wipe down every day. TThen you only have a smelly top part left.

Or, have both floors adjusted so that you can tuck fleece around the whole bottom plate. I see people doing this with dog crates they use for bunnies. The bottom plate is then removable and they can fold the fleece around it so that there are no exposes edges and the fleece stays in place. Under the fleece you could then do something waterproof so that it doesn’t soak to the wood and again I suppose you’d have to replace the fleece daily to prevent smell build up. I saw it in a Youtube video but sadly I can’t find it anymore. Be sure to check out Youtube or Pinterest though, you may find inspiration and solutions there.

How old is your bunny and are they spayed/neutered? When bunnies reach puberty (between 4 months to a year) they may lose their litter habits and start peeing because of their hormones. This may or may not be temporary until the bunny reaches adulthood and if the bunny continues to do this as an adult spaying or neutering may solve the issue.