LOL ... always been one of the hazards of owning a truck. However, it is my observation that the older you, and your friends, get the less of a problem it becomes. Last thing my friends want to do these days is to lift something into the bed of a pickup.
wait a few years... their kids will borrow it to help the folks.. lol
simple/fast
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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