Alternate Vent Magnet Mod Method (AVMMM)
Mark Whitehead (of 100k Rav4EV miles fame) decided to get all fancy and improve on the Darell Magnet Method™. What he came up with is pretty slick!
The magnets are 1/2" wide, 1/8" thick with a 1/8" hole. Magnets from Emovendo.net or eBay (sample item# 160118398447)
I used stainless steel screws 4-40 x 1/2" (#3712) with SS #4 and #6 washers (#5563 or #2942) and SS 4-40 Hex lock nuts with nylon insert (#4107). I used the #6 washer on the inside of the plastic but I think a #8 would be better.
All of the screws etc. are from BoltDepot.com (their numbers in "()" ). Lowes has these too.
Sequence is screw head with size 4 washer, then magnet on outside of vent clip plastic. Underneath would be size 8 washer, maybe another size 4 (I did not do this) and then the nut. With the locking nylon ring, no need for Loctite.
I put a magnet on each clip holder except the one for the tether. I made the tether out of 1/16 inch metal cable.
I think you had a smoother finish on the car and I have a slight risk of scratches from the screw heads on the paint if I am not careful. But I liked not having to mess with the glue. Overall I think either way works well. My magnets are not as strong as yours (I bought both) but with no plastic in between to affect the attraction - the hold should be about the same.
Link to assembly diagram PDF
Assembled to the vent stand-off: