Quick update. Handles added to the front and back of the box. First pic is lacquered and reassembled without the unnecessary frame bits. Second pic is box mounted on the bike ready for the first ride.
So far done a 10-ish mile test ride, lugged some junk round to the garage, a bit of pottering around and a bit of a hilly 16 mile ride to Virginia Water with 3-year old onboard, picnic, blanket, cushions etc. Seems to actually work best without the seat base and a load of cushions instead. Will try and get some better pics before the box gets too beaten up.