Dimensions are flexible, within limits: 41 in. x 16 in. x 5 in. is a target. This will make the cabinet “invisible” behind the fold-down table. A 5 in. depth will result in the table protruding a minimal distance aft beyond the mast. This depth will also allow for storage of plates and bowls as well as glasses depending on what you have. This is essentially unused volume and will not diminish usable space in the cabin. With the table fully deployed, the seating is slightly improved as the port-forward seat is neither crowded by the bulkhead nor hidden by the mast.
Choosing your internal dimensions is a matter of your needs. Measure your plates, bowls and glasses and refer to the photographs for ideas. Don’t forget to allow for the thickness of the shelves.

N.B. For these dimensions you will have to remove the fiddle from the port inside table. If you choose not to do so you will either have to make the cabinet narrower, to fit inside the fiddle when the table is raised or install a thicker block at the bottom so the table can “stand off” the additional thickness of the fiddle. You could plane the fiddle down to reduce the stand-off. I removed the fiddle but it did cause some damage to the table.

Construction

I used ¾” teak plywood for the sides and ½” teak plywood for the top four shelves (including the top). The shelves were made from the doors separating the head from the forepeak. I had removed them, having decided they did not suit my needs. The joints are glued/screwed dadoes, but screwed butt joints would be satisfactory since the loads are minimal. Edges were covered with iron-on mahogany edge-banding. Treated with teak oil, the appearance was acceptable. I used strips of ¾” teak a bit less than ¼’ thick for the rails.

Mounting the cabinet requires two people: one to hold the cabinet to the bulkhead and align the inboard edge with the mast and the other to mark the bulkhead inside the cabinet near the top and bottom. Then measure outwards, port and starboard half the thickness of the wood you used for the sides of the cabinet. I marked two spots on each side and drilled from the cabin side into the head. After screwing the cabinet to the bulkhead from the head side, with the helper holding the cabinet, I drew a line on the head side of the bulkhead to locate additional screws.

The existing piano hinge of the table can be screwed into the bottom block. You may need to use larger screws; there will be a greater load on the hinge - #8 x ¾” should work. Detach the closure hook from the bulkhead and attach it to the cabinet side so that it will mate with the socket on the table edge. Fit it so that some pressure is required to close it, to reduce movement in a seaway.