My first thought is that if the existing rack is does not provide any easy
way to secure a sheet of plywood, it won't be real easy to secure your box
either. Your box idea is going to be pretty heavy for a car roof and unless
you have a pretty long roof on the car, you're going to have a lot of
unsupported surface in a box that long. You don't say what the vehicle is,
but at first blush it seems like you're over engineering your solution.
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