I use a Velox cover for my B-17. Light, strechy tight fit, full waterproof even for a 15 hour drenching rain ride (unlike the Brooks house brand). For $8 and I'm just now looking to replace it after 5 years of constant use, I'd say I've gotten my money's worth.
Sadly, it appears they do not make this version any more and I have to pick something new. I don't know about the Rub-ber, though. Is "biodegradeable" really something I want in a saddle cover? I'd prefer something that has low potential for degrading.