I also didn't want K to have free run of the basement area. So I had to come up with an enclosure that would keep him from getting into things he probably shouldn't be messing with, and I didn't want him to be able to get to the outside door in case I opened it and he jumped out. I still needed to be able to access the litter box from the outside basement door for cleaning. I came up with a hinged wall that gives a large enough area for the litter box, and can be unhooked from the wall just inside the outside door and swung to the side so I can get to the box to clean it.
The other benefit of the hinged wall is the whole thing will fold flat against the inside of the storage space, allowing me access to the hot water heater in case that needs repairs.
The last bit of the project involved installing the cat flap. I had to build a framework to support the cat flap and cut a hole into the interior of the camper.After that it was a simple matter of screwing the cat flap to the wall, and the project was done.