Boy, I would have never imagine so much interest in covering over these vents.

The sheet magnets on the site Neil 21 found is very interesting because the stuff appears to be very inexpensive. I plugged in a six by twelve inch size of their .030 thick material and it was a couple bucks each, less any shipping.

Knowing what magnetic strength "number" to use on the site, from a computer screen, told me nothing. Might help if I knew, What a refrigerators magnet strength was? I know they are thicker but not real strong. That type of strength would probably be fine if it has even and flat contact with the car.

The next trick is putting holes in there like a screen has or just where we need them. One, would need to use a 1/16 inch gasket hole punch to get clean round holes in this kind of material to get even looking holes that makes it have a professional finished look.
The whole project could be with a cute looking template that has nice rounded corners and contour.

I think the color is always going to be a contention if one wants a permanent attachment. Putting in some even looking holes makes it look more professional. May have to use SEM flat black automotive paint for some class.
Eye appeal, is sales appeal. I was told that by a Baker, friend.

Your cover looks like it does the job. You must be saying ugly, for its thickness, if you are focusing on the frame. That frames shape looks good to me as its very functional to utilize the wire ties.
Your design deals with the stability for the front long side. My idea has no border and I am not sure I need to deal with it for my purposes.

Neil's looks thinner with use of the tape. Magnetic could solve everything as long as it cannot slip. Maybe the 0.020 thick material will still fit through the slitted rubber hose idea and be flexible enough to be worth pursuing.

All these ideas could turn into a good soup yet!

Ps. I just read the other posts about the CAD and laser cut, wow! I thought it was nice now I get why.
Are you sure it was laser cut or was it a water jet machine? Both techniques make it easier that what I imagined it to be made by hand or even on my manual milling machine.

Phil