happyjack, I had a hovercraft that needed a higher cover similar to what you describe. I extended the legs, but found that overall it was too wobbly to trust in a wind. The more expensive heavy duty "canvas" garages are probably what you would need instead. They have about 2" dia. heavy steel tubing and are built for snow loads, but a bit pricier and more permanent and come in heights to accomodate RVs. I'm currently using the tyvek cover available from Camping World. I got the one for up to 21' and it fits my 211 fine. A little extra material where the cabover should be, but there's a tie-across to pull it in. It is waterproof and white on top, and breathes through gray-colored material on the sides.