As promised here’s a (not very good) close-up of a cabin hook that latches into the bottom of the hatch/topbox. There’s another on the other side. The ‘staple’ part of the hook is fitted into the base of the box from above leaving the base of the box smooth. The hook goes into the slot.
Here’s the top of the door frame. On a conventional hatch/door set up the doors extend up to the roof line and meet the underside of the hatch. Not practical here because the roof extends about 120mm past the rear bulkhead and the hatch/box goes right to the back. If the doors were taller they would open under the overhang. If the hatch/box had a vertical bit above the closed door position it would prevent the hatch opening fully.
The solution is to mount the top of the door frame on hinged arms the hang from the rear edge of the box. When the hatch is closed as above the arms swing forwards under the box providing a rebated frame top for the doors to close against.
When the doors are open and the box pushed forward (open) the frame top section swings back and up against the rear of the box (right). Needs varnish and draught-proofing but after many months it’s nearly done.
Next, fitting the spare gas bottle locker and paneling inside the rear bulkhead.