I used to have a Cycle Shell brand. It was the first portable type cover available.
It fact I bought their prototype which they heavily discounted for me...
http://www.cycleshell.com/Please note: when collapsed & folded for storage with integrated floor they are quite cumbersome.
I used mine for 2 years then sold it when I had a larger garage for the bike.
It protected through the winter and summer storms quite well with little dust accumulation.
However, they can act as a humidor too in rainy climates!