Oilcloth, mate - oilcloth!
I drive an old Land-Rover - and its canvas top gets a beating from sun/rain/UV/whatever. What we typically do with them is lay them out on a HOT day and coat them with linseed oil. The heavy cotton duck will soak up the linseed oil, which when dry creates a waterproof cloth that used to be known as oilcloth.
A good cotton duck is essentially waterproof, but the oil helps a lot as well. I've never gotten soaked driving my 109 in anything up to and including hurricanes (through the Notch up near Stowe, VT a long time ago {shudder}).
Alan