> it sounds simple but it's really not
Putting a man on the moon is complicated. Storing some light gear on the roof of a vehicle in a diy version of a ubiquitous commercial product, not so much.
And thanks, using a metal roof storage rack as the base is an excellent idea. As is an aluminum storage box but I worry about its weight. Hmm.
And sorry I should have mentioned, the rv is a 24' 1989 Jamee. The roof is reinforced in the rear area where I plan to put these. And nothing I plan to store is particularly heavy (wetsuits, scuba gear, kite gear, surf gear, life jackets, NorCal fishing clothes), just bulky. Far too bulky for any space I have inside.
My main concern is finding a box made of reasonably resilient plastic. In my experience polyethylene does well, which is why they make kayaks out of it. Not sure if they make any bins out of PE but that would certainly do it.