Providing a dig box will NOT keep him out of the cat box - rabbits will not be deterred by anything short of blocking access.
When I tried to free-range my bunnies, they insisted on camping out in the cat box an awful lot, though thankfully they didn't dig at all. I worried a good bit about the cat poop (and their tendency to eat cat food, both of which contributed significantly to my decision to go back to keeping them in a massive x-pen), but had NO concerns about the litter causing problems because I use the same bunny-safe wood pellets in the cat box that I use in the rabbit boxes. Both our young cat and our stubborn-as-hell 15 year old cat didn't care a bit when I switched them from clay litter to the wood pellets (some cats are extremely picky, but the vast majority have no issue with the switch). They actually make a cat litter called "feline pine" that's the same as what I use, except that I buy pellets marketed as horse bedding from a feed store for a fraction of the price that pet stores charge for the "cat" stuff
.
As for a dig box, there are tons of different options. I opted for this:
I really wanted a
pool with a slide, but couldn't find one cheaper than $35; I ended up cheaping out and going for a
$13 pool. The site I bought it from is a distributor who sells to numerous retail businesses - they offer free site-to-store shipping to any store they distribute to, so I didn't have to pay shipping. I brought a power drill, put a hole in the rim of the pool on either side and used rope to secure it to the roof of my sports car. This was in December; I got a ton of weird looks on the way home even though it was 80F+.
I filled it with a mix of topsoil, hardwood mulch and sand; Home Depot sells two types of play sand and the first was dusty enough that I didn't feel it was safe but the other was much better. I used non-toxic, egg-based tempera paint to dye the sand green
. (There's not nearly as much sand as you might think, as I hadn't mixed it in when I took the picture so it was all on top.)
The one issue with dig boxes, regardless of what you make them out of or fill them with, is that bunnies almost always use them as pee boxes in addition to digging in them... so if you make a dig box, be prepared to have to de-peeify it on a regular basis ><