I've read that Tyvek makes for a great cover material, if you're up to making a custom-fit one and don't necessarily care that much about looks. Tough, water-resistant and breathes well. My 550 cover is getting a bit tattered (stored inside during the winter...) and I don't have one for the 175, so I've thought of making a couple for myself. Just make sure you make it so it 'vents' in the right direction.