You could try a company called Copter Covers, I know the company name doesn't sound like it but they can provide covers for fixed wing aircraft, too. I remember the guy who runs the company being very helpful as I tried to talk him out of gizzits at Helitech.
I honestly don't know what the covers are like or how expensive they may be but the free baseball cap keeps me warm and dry.
www.coptercovers.com/