Seen a lot of things used as a rain cover by commuters. Everything from plastic grocery bags to shower caps. Best to date is something put out by bikecity.ca called the rub-ber.

https://www.bikecity.ca/en/products/r...eat-cover.html

Black latex is a stretch fit over the seat, is thick enough to stand up to abuse and stretchy enough to fit a variety of seats. Nothing to slip or to tighten up.

Just thought I'd mention it. You can buy direct apparently. Have one in my hot little hands as I type this but haven't found an LBS here in Montreal that that carries them yet.

Burton,

Do you have a link? I tried to go to bikecity.com, but it doesn't appear that the page is anything other than a landing page. I've been looking for something just like this.

There are people on etsy making them out of their homes, I got some custom cut for my wife daughter and son. The 1st 2 were good but the one for my son came out too small. I am too lazy and they were cheap enough that I have not returned it and he is back on the plastic grocery bag.

I've adopted the Bladder bag out of the 5L wine box
for a winter foul weather saddle cover
and, a spring document clip holds it in place, nicely..

OK I've added the link to the original post and confirmed it works - at least for now:

https://www.bikecity.ca/en/products/r...eat-cover.html

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.

You're kidding right? Latex mattresses typically last 20 years, latex tubes till you puncture them and the latex backing on carpet usually outlasrs the carpet. Technically your whole bike may not be biodegradable if its made of metal, it'll still corrode if left to the elements.

A 4" chunk of latex foam Vs a Membrane maybe 1mm thick.. not really comparable.