If I understand you properly, you're looking for some sort of a gizmo that will make a little access hatch or pass through for your van body, but can be sealed when not in use to keep the weather out.
One possibility might be a
deck pipe or some sort of a
deck plate for a boat.
Another possibility might be to assemble something from pipe (PVC or metallic) using a screw in cap/clean out plug to close it. If you have a cap on both sides, I'd drill a little hole in the inner one to prevent the possibility of air pressure causing difficulty in closing or opening it.