I currently have a 2004 Z4 that I am trading in on an ordered Z4 M40i.
I have tried many car covers, including the one that BMW offered.
My car stays in a covered carport, so the cover is mainly for dust. I have ALWAYS had trouble with the thing blowing off.
I recently got this one from Carid.
Covercraft® C16451PG - WeatherShield™ HP Gray Custom Car Cover more details on -
https://www.carid.com/my-account/order/34297182/
I live just north of New Orleans. My area has been through 2 hurricanes recently--luckily winds only got up about 60 MPR here. Neither time did the cover come off. Its very form fitting and leaves nothing on the car or top when I take it off. Its a little more than I wanted to spend, but its worth every penny.
I've already ordered a new one for my new M40i.