I'm a little confused by the scratching post, but whatever they use to dig up should be great.
Sorry, I've been out of town and didn't see your reply until today.... so how do your buns like the new litter tray?

If indeed the scratching post is what your buns use to dig on, another very good and maybe less expensive idea is to use new carpet scraps and put those down (anchored) in the corners or places your bunny likes to dig. I've also gone to carpet remanent/discount stores and paid a dollar each for their discontinued sample squares....they are usually a pretty good size...15 x 22" maybe??? I've also bought a remanent piece (5 foot) and then cut it up using one of those razor cutter to the size I need.
If you are unable to anchor the square down somehow (cuz when digging the buns sometimes move the carpet piece and then dig where they aren't supposed to--I have hardwood floors but others I know have "real" carpeting), I've also use heavy duty/industrial stapler and stapled a remanent to a piece of plywood. That's almost too heavy for them to move around.
Just another trick of the trade. Hope this helps.