I use Weathershield - unfortunately, it does not keep
the car totally dry underneath. I have dew condensation
in the morning and sometimes that will even seep through the cover. (I think I wash the cover one too many times and the cover becomes less water repellant.)
Perhaps, you should try Bill D's method.
Not cheap, though. I personally will probably go crazy - putting
on and taking off car covers are not my favorite tasks -
even when it takes me about 30 seconds to do the cover.
The best price I have seen on web is from
www.ajusa.com.
(free shipping, too)
Good luck.