by garbarrage » Tue May 08, 2012 10:35 pm
Have never seen anything that fits the bill here, but that's not to say that they don't exist. Bring your car with roof box and boards (maybe inside the car) to a fabricator/welder/engineering place. We use them a lot in work for specialized equipment stowage on our vehicles. They are often a lot cheaper than you may think, and usually find coming up with novel solutions a refreshing change from their usual work.
I'm assuming your roofbox attaches to a normal roofrack. Maybe they can bodge together a couple of upright brackets that fit on to each rung or your roof rack to attach your boards to. Good fabricators often astound me with the ingenious solutions they come up with.