SNKBTE,
As I don't have a roof / softop I wanted a car cover that could repell the heaviest storm (which we sometimes get up here - not as often these days) without scratching the car.
I decided to import one from the States. See link
http://www.tprmag.com/misc/calcarcover/material.html and bought the stormweave blue cover.
It is absolutely brilliant. Repells the water 100% without the car sweating and does not scratch the car. The material looks paper thin but is as strong as.
I think I bought a medium sized one and it fits the car well - no stress on the windscreen.
The covers goes on in seconds and rolls up into a bag for storage in the boot.
Not cheap, but highly recommended.