I have been using the plastic ones from GoWesty with pretty good success.
I have been driving with them without any problems flying off - and going
80 in both hot and cold conditions for the past 3 years since I discovered
them.  Yes, the plastic ones do get brittle and crack after a while, and if
you don't replace it, it breaks apart.  But I find it lasts about 10-12
months, in SoCal conditions that is, before needing replacement.  Sometimes
I have noticed that you'll see some condensation and I simply remove the
cover for a couple days.  I know they are there for washing, but agree that
they keep stuff out so I just keep it on.  I think they are $10 or so at
GoWesty... and I think Bus Depot sells them as well.  I wanna try that
Rubbermaid one a listee wrote about #12 I believe.  So far they have not
been ripped off, and I drive & park in Vanagon-world where you see tons of
them here.  (Hope by admitting this it doesn't change my luck...)

Joe Gawzner
90 Syncro Westy Vanaru