In reply to kingjam:
Agree with using a bit of plywood as a back board.
Another way is to mount two vertical wood battons above the door (say 2 x 3) and then screw the ply board into those. This helps bring the board out that bit further from the door. And if the wall is slightly uneven it won't matter. And if the screws from the fingerboard into the ply go a bit deeper than the plywood they won't go into the wall behind.