https://www.google.com/search?q=car+...gxBlsQ_AUIBygC
Some use luggage rack crossbars- no holes but not aero.
Some use magnets- no holes and good aero.
Re: x-wind: I was thinking of pivoting the panel from the front edge like a rudder, then perhaps limiting the movement to, say, 45 deg either side, and dampening the swing somehow- rubbery friction washers on the pivot mast and/or bungees or?
You need this:
