You'll see some good discussion of covers if you search. My Boxsters are outside unless I'm working on them. The best cover I've had was a custom fit cover made with "Noah" material. I've tried lots of others and none were and durable or protected the car as well.
A good buy on a lightly used Porsche outdoor cover would be a great choice too.
Right now I have some cheap covers:
Duck Covers, 170" x 60" x 48" They are the best
CHEAP cover I've had. Not custom fit, but close, and no mirror pockets.
Other thoughts: Buy a Sun Joe power washer - do not put a cover on a dirty (or wet) car.
Get a smaller
window & roof cover:
Great for sun and easy to use and remove for a daily driver - Full covers can be a hassle for daily use.
-Formerly of Clarksville (2 tours, and an ex-wife still in town).