The spell requires "must be contiguous with at least one other panel" and that you can "crudely shape the wall to create crenellations, battlements, and so on."
Adding a roof to make an enclosed space seems to be 'permissible' within the limitations, but is it allowed since that's not really a wall?
In short: YES
The wall can have any shape you desire, though it can’t occupy the same space as a creature or object. The wall doesn’t need to be vertical or rest on any firm foundation. It must, however, merge with and be solidly supported by existing stone. Thus, you can use this spell to bridge a chasm or create a ramp.
This is a spell that is often overlooked as one of the most utilitarian there is. As long as the creation is simple (as in not elaborate), then yes. I have always thought of using that spell as something to do with Graham Crackers: box-shape and simple.
