That is true about the fragility of the vents, but my crew and I have a process of stringing a large bumper horizontally from rear cleat to mid (with a loop around the railing) whereas the rear fender sits just in front of the vent. Going on the third summer I think and haven't broken one yet
Ideally one wouldn't have to worry about them standing proud of the side of the hull... That's a good solution you've come up with with the aluminum angle.