The back is a single piece, designed to fit exactly between the posts. I have my TARDIS standing in a doorway, so the back is normally removed. It can be attached with hinges however. If permanently attached, I’d probably add some beading around the edges to prevent light filtering through the crack between post and door. If you wanted to go for extra rigidity, I guess you could use the protruding 2x8 technique that I used to connect the sides to the arches. (See the description of the sides for joining instructions if you do that.)

The back is a single piece of ½” MDO plywood, 36” by 76” as seen on the exterior plans. The surface decoration is ½” MDF fibreboard. This stuff produces nasty carcinogenic dust when cut with a table saw, so use a mask, extractor and do it outside if you can.

I cut the 3” vertical MDF boards to the full height (76”), 3 ½” wide at the back using a 45 degree cut on the table saw.

The cross pieces are bevelled inward on the long edge and outward on the short edge so that they meet the full height boards at an angle. I cut them slightly longer than necessary then trimmed them to the exact width, as even a 1/16” gap is very obvious. I still ended up using spackle to fill a few gaps. I glued the MDF boards directly onto the MDO. Since they aren’t structural I’m not expecting this to cause a problem, provided the glue holds (!).

I glued a 1 ½” by 3/8” piece of wood down the centre of the back to resemble the doorjamb seen on the front.