That all depends on the projector. Obviously, if the air intake is on the back of the projector, you may want to rethink your solution. Likewise, the hot air must be able to exhaust from the projector case. Too little clearance could prevent the hot air from escaping from around the projector.
Projectors do not like overheating very much. I recommend that you verify the air intakes and exhausts before finalizing your mounting solution!