I installed 2 vents on our Bus and dealt with the curve issue by adding extra strips of that waterproof putty tape (made for installing vents & such on RV’s ) on the ‘high’ sides of the vent .
Basically, I built up the waterproof tape until I had a reasonably level base to work with…
Then I metal screwed the vent down just enough to get a tight bond …but not so much as to bend the vent body.
Hasn’t leaked a drop ….in the rainiest place you’ll ever see in the US.
Seems to work great.