I've actually seen some pretty neat ideas on Pinterest and the like that consisted of a kitty litter pan, a heavy gauge hire that was lifted about 2" off the bottom, and a little hay rack at one end (to encourage the bunny to stay put while it does its business?). The idea is that you can add pine shavings or pellets to the bottom of the tray to soak up more of the pee, but the bunny won't be standing directly on it.
If you have a deep box, dried rabbit poop works well. Pee flows down to the bottom while poop on top stays dry. It only stinks when you empty the box and the urine is exposed.