You know, I have seen this done before... Is this cabinet going to be in the utility or bathroom? Only asking because it might make a difference in the style. Also, if you plan on storing all the kitty related stuff in there too, then it would have to be a fair sized cabinet, right? Could be a tall, shelved cabinet. I mean, if the cabinet is wide enough to accomodate a standard sized kitty box (assuming you are using a standard sized box) then you could just buy a stock base cabinet, maybe with double doors, and install a kitty door in one of the doors,but you would also probably want to vent it somehow and maybe provide lighting? Do kitties need some light to be comfortable doing their business? Just throwing out thoughts off the cuff here so take them for what they are worth... Good luck!
I have kitties, dogs, and horses. But my kitties have a small room to themselves right now...
You can see stuff at my blog about some of the critters, as well as others critters.
LJ