Goodalls make good boat covers. You can get a light duty material called Canvacon or a Heavy Duty Acrylic canvas. The light duty one would set you back around $300 and the heavy duty around the $650 mark. (Ring to make sure)

Goodall Sails (AHPC) 03 5443 6910 or ahpcinfo(at)ahpc.com.au

If you wanted to make your own you should be able to find Canvacon at local loft or marine supplies store. You mostly see Canvacon as boom covers on big boats with their name on it, while moored.

Regards

Matt