You'll have to clean it very frequently, as they pee and poop a lot where they eat, and their butts will be pretty much in the same place the whole time they're in there. Also, they may decide they like sleeping in there, and that's not a good thing. They'll be lying in their own waste and breathing the fumes.
Couldn't you just find something with higher sides and cut a hole for a door, and ditch the roof?