Hi! I'm putting together a dig box for my bunnies. It will be outside so they can make all the mess they want. I already got a huge plastic bin (similar to this) and I'm considering using either soil (plain, clean soil with no fertilizers or fillers) or sand. Which do you think would be better? I read somewhere that rabbits love to dig in sand, but it seems like it would be frustrating to dig and not really be able to make a proper hole. And since they're indoor rabbits, which would be easier to get out of their fur when it's time to go back inside (they're both short-haired)?