I used this homemade boat rack on the old camper to pack my 14 footer and my canoe for many years. Not the best pictures but all I could come up with. Aluminum flatbar, a couple little pieces of aluminum angle, and two 1 1/2 aluminum tubes.
There is one piece of angle pop riveted to the flat bar so it sits flat on top of the roof, the flat bar is attached to the camper with some 1/4 in lag bolts. The second little piece of angle is attached to the square tube and to the flat bar at the appropriate height.