http://forums.rabbitrehome.org.uk/showthread.php?t=225257
I would put some newspaper down and some vet beds/fleece blankets with a towel or two underneath. Fleece (and vet bed) wicks away moisture, so if there is a towel underneath and newspaper this will absorb it nicely. Sawdust is probably a bit dusty for his wound immediately after his op.
You could also use lots of lovely SOFT hay if you prefer, but I would use a paper based litter underneath for absorbency that is less dusty than sawdust.
You will probably find for mess reasons and convenience just using some towels and a blanket will be easier.
Good luck
:wave:
ETA
I would also ask the vet for the weekend/out of hours contact no.'s for an emergency vet.... just in case. Generally it might be better to have surgery done mid-week but my vets are open Sat and Sun normal working hours anyway, some vets are. Castrations are straight forward but it's best to be prepared rather than trying to find the no. in a panic. Don't forget the pain relief!