Even though it's sunny today, I know the rain is around the corner. It will be my first winter attempting to bike to yoga. Any suggestions on a waterproof way to carry a yoga mat?

Thanks!

Plastic bag and bungee cords?

You may have already looked but, a search for "waterproof yoga mat bag" (no quotes though) will yield several results. My wife sews her own yoga mat bags. If you're crafty, you may even be able to make your own.

Plastic bag and bungee cords?

This is what I have done in the rain. Either one regular sized plastic bag over each end if it will fit or one large garbage bag. Strapped to the end of my rack with bungee cords, I've had no problems getting it to Yoga class.