Yes I know that feeling, driving down south for 3 hours with an ever increasingly annoyed wife! Extra straps help, but only if in the right position and the right position often moves !
I ended up with a unique solution. I bought a second board in carbon, so now I use the older,longer board with the neo camo deck, face down on the roof racks, no cover and then put the more expensive carbon board on top in its cover.
A somewhat expensive solution but the upside is no more flapping covers and two boards instead of one