Tonneau Cover

I have a 5.5' box as well. Any time I've had to transport 8' sheets of anything I leave the tailgate up. Less chance of the sheets sliding out when it is angled up and no need to strap it in. My tonneau cover, although foldable, sits loose on top and doesn't move.

I bought a used cover and had to get it home on the truck, now i had the tail gate up, but this will also apply to you. I just used a ratchet tiedown to hold the cover on. I also have a crossbed box in the bed and it pushed the cover out back by another ft or so. No issues in almost 200 miles.

So if you have concerns lock it down with the original hardware and use a tiedown to make up for peace of mind. How do you plan to secure the plywood?

Depends entirely on how the particular brand of cover you have is designed. Easy way to tell is to lower the tail gate, lock the tonneau and see if you can lift it back up without unlatching it. MOST of the latch into the sides NOT the tail gate but there could be some exceptions.

My Tonneau cover latches to the side rails and not the tailgate. I would maybe lock them so they don't vibrate open.