21 3/8 inches front to back, hinges screw through surface with T-nuts. 3/8" birch plywood, 7 ply, shaped and drilled. This is correct for cars in which the wooden spare wheel cover was NOT covered by an aluminum skin.
Original factory part number BD9372 (wood only).
If the spare wheel cover is covered by an aluminum skin and the hinges screw through the side, see listing for XW8 in Related Products below.