I had a 1968
Tradewind and lost the rear utility hatch (approx. 18" high X 48" long) on the way home from a camping trip.
Inland RV was the only place I found that made the part and it cost a pretty penny.
I immediately bought a marine battery holder system that included two nylon tire down clips screwed into the bottom of the battery compartment floor, combined with a nylon strap around the top of the battery, snugged down tight. I added two cables screwed to the hatch innerface at curb and roadsides, then to the hatch frame on the outerside. I also included two barrel slides latches to each outside corner of the hatch.
It amounted to $45.00 worth of hardware to secure $400.00 of battery and door.