Flooring for outdoor rabbit pen?

I built an 8x8 pen outside under my back deck. It has a roof and it is mostly weather proof, but I'm wondering what to use on the floor. It is a hard wood floor, that I don't think would be very comfortable. I also don't want to use loose bedding unless I have to (really messy and expensive).

Tl;dr On a hard wooden floor, do I need to put a different type of floor, if so, what?

When I had an outdoor area for my rabbits, I used a horse stall mat. It worked great except it would turn the bottom of their feet dark. That was years ago, so maybe they make them better now. For my rabbit cages I use thin non-skid rubber mats. They work but the rubber has a texture that is hard to clean unless I remove them and hose them off. I have one rabbit that chews on the rubber, so I use cardboard in her cage, which I have to keep replacing because she chews that up too.

Loose bedding just leads to confusion about litter box habits anyway. Most rabbits are fine on a hard floor; exceptions being if the rabbit develops sore hocks or seems bothered by the slipperiness of the surface. If you want something that gives more traction in some areas, one option is a vinyl/plastic mat or runner from a hardware store. Cardboard is cheap and effective if it doesn't get peed on (since the pee could soak in and effect the flooring). If you want to offer a more cushion-y place to sleep, you can try a cheap fleece cat bed (or loosely piled fleece fabric).

“Mostly weatherproof”? This can cause a lot of problems depending on the type of rabbit, temperature outside, etc. Snakes?

Why are your buns outside? Just curious. There is an infinite amount of information and an infinite amount of people here who have amazing ideas on indoor setups if you need any suggestions.

Sorry now there's a bottom on the run, so its fully weather proof. Don't worry lol. Anyway they're outside because of allergies, and there aren't any spare rooms that we can dedicate to the rabbit

If your rabbits are litter box trained then a solid wood floor should be okay. If they aren’t, and your rabbits start peeing on the wood floor, then this will allow bacteria to grow, which isn’t good for the rabbits.