I have found that many projectors have a vertical lenses shift that is much larger in one direction than another, by this I mean if you ceiling mount it the lens shift will be greatest towards the floor and if you place right side up on a surface the lens shift will be greatest towards the ceiling.

There have been a couple of times I have had to flip a projector upside down inside the enclosure in order to get the image down to where it was needed. Ideally you would use a projector mount on the "ceiling" of the enclosure in order to more easily keep the projector level and square with the screen wall.