If it were me i would take a sheet of plywood and screw it to the underside of the deck above the quad. Add a 1" spacer to the one side so the water will drain away from the deck. If you wanted to get creative you could also build some walls out of deck boards or something that would look nice. Even some tight privacy fence would work good.

 

My old cover was getting torn up so I had to buy a new one. I got one from Classic Accessories. The elastic on the bottom was too loose so I pulled part of it out and tied it in a knot so it's about a foot shorter. Now it stays on except for the very worst wind storms. Too bad my seat is in such bad shape from the sun shining on it for 5 years.

 

I think I over-stated how good the cover stays on. I looked outside and it was completely off and I didn't think the wind was blowing that hard. Weather.com says the wind is 28 MPH but that probably doesn't include gusting. I may have to tighten up the elastic again.