Mark, how are your nucs packaged?
I don't think the sheet metal would flap, because the way my trailer is set up, there's a 6 foot enclosed space on it, and behind that a 4 foot open space. I planned to strap down the nucs at the tail end of the open space, put 2X blocks to separate the sheet metal from the nuc tops, add the roofing panel, then ratchet strap a 4 foot long hive on top of that.
I wonder if I bought a garden sprayer, filled it with water and ice, and sprayed them down every time I stopped, if that would help.
I've also thought about leaving in the evening, so that I'd only have one day of travel, and two nights. I assume the nights are easier on the bees.