I thought I was the only one w/ two of those laying around!That's why I've had a pair of the vents laying around here for two years and haven't installed them. I've seen some that installed nice and some that just looked like crap.
The two sided tape won't hold it down enough. If it were me, I'd cut as much of the vent off as I could so it would be more pliable, and epoxy it in place. You can get some small clamps in there to hold it down while the epoxy dries.
I've decided to just send my covers to Ducks...whenever I can figure out what I want him to do.