I installed the L-bracket inside the vent with an small ring shaped earth magnet and did the same on the flapper. The screws that attach the magnet give it some distance so the strength isn't too strong. After a day of 60km/hr gusts, we hear no more flapping! I'm waiting for the 100km/hr wind gust now to really test the thing.
There is 1 drawback, we do have to turn the hoodfan on "high" to force the vent to open, but afterwards, you can set it to "low" once it opens. You can hear the rush of air push the vent open. Also, when it closes, you can hear a slight tap of the screws touching each other.
A good and cheap and simple solution compared to the Hoyme Motorized damper I was contemplating on getting.