Sailrite.com is a good source for materials and thread for your project. I'd agree that Sunbrella is probably the way to go. If you want to make a good looking cover, I'd go with a fairly well shaped, fitted cover, that follows the contours of the pedestal and wheel, and has either a drawstring, snap flap, or velcro closure system so that it doesn't flap around.
The side for the wheel should be a U-shaped piece that is a bit wider than the wheel and tall enough to reach the bottom of the pedestal. The other side will be a bit more complicated, as it has to be a basic U-shaped piece with a section cut out and replaced with a fitted cover for the pedestal.
Overall, it's not that difficult a piece to make. Any questions, drop me a note.