Strap the luggage down, leave the tailgate down, strap the Board & screen (you can even build a box/lid with 4 boards, a hammer and 40 3" nails). To really secure your luggage, tarp it, then put a cargo net over the bags, and strap the whole works.
If your ratchet straps are secured properly, nothing will fall out. Take a look at a semi hauling drilling pipe some day. Granted, you are not securing 20,000 pounds of pipe, you are securing a couple of a hundred pounds, therefore the straps can be a lot smaller.
$50 for the materials down there is cheaper than $150.