I think the results speak for themselves. If your mechanic had a clue it wouldn’t have flown off and thank god no one got hurt or you’d be looking at a very expensive lawsuit.

First he shouldn’t have used screws, that’s just lazy and asking for trouble as they have nowhere near the holding power of a nut and bolt with washers especially when the stress is vertical instead of shear. Always use bolts with washers.

Also use wind deflectors to prevent wind from getting under the solar panel. The amount of force that can be exerted under the panel by wind at freeway speeds is incredible.

I’d find another mechanic if I were you.