Great grid system (though cookie racks can also work) -
http://rabbitsonline.net/view_topic.php?id=53690&forum_id=93 (check 1/3 of the way down the first page); it thwarts digging and prevents them from having to stand in soiled litter box substrate.
I used carabiners instead of shower curtain hooks when my girls were moving their litter box around the condo and I used a scratch awl because I didn't own a drill at the time, but other than that, my method was the same as PaGal's - I put two holes per side in two sides (I did one end/one side since I had the litter box in a corner) and used zipties to attach the carabiners so that I didn't have to snip/replace zipties every time I took out the litter box.
If he manages to thwart whatever grid set-up you use, zipties can solve that problem as well (they're like duct tape - the solution to a *wide* variety of issues!).
I've got a HUGE plastic storage box as my bunnies' litter box right now, actually - not because they dig, but because they kept peeing over the side of their old one (even though it was quite roomy) and I wanted something even bigger. I took an old bin cage made for a hamster that had a window about 6'' high and 17'' long cut out of the middle of one of the long sides - I just removed the hardware cloth that I had bolted to the window and it was ready to go
. (Oh, and with a fairly large box and wood pellets as substrate, I've found that it's too heavy for them to push around, let alone flip on its side so I no longer have to tie the box down.)
One option for a large box that's fairly deep but wouldn't be overly tall like a storage box or a covered litter box is an "organizer tray" for an 18 gallon storage tub - Walmart sells them for $3, which is cheaper than a large cat litter box.