i don't want to sound pessimistic, but i think there needs to be quite a lot of tension to hold them flat against the car at 55 mph. at that speed, if they lift away just a little, they'll be off in an instant.
i think you'll be okay for the turns, since you'll most likely be below speeds that would be a problem (on/off ramps notwithstanding).
Someone got a load of coroplast in the windshield!! LOL Didn't you see them fly off???
You may want to add an overlapping piece of material over the leading edge of the cover taped to the front bumper/fender in front of the wheel so that the leading edge of the wheel cover can not scoop air into the wheel well. You may want to make it a two part wheel cover allowing the front part to hinge on the leading edge on the fender ahead of the wheel and keep a vertical overlaping seam at the center of the wheel so that only the front edge opens up when the front edge of the tire sticks out and the rear edge opens up when the rear edge of the tire sticks out.
Think in terms of the air flow not gravity for your hinges. Also you may only have to cover the edges of the wheel well close to the tire but not over the tire if you have smooth wheel covers. This way the cover would not have to move when the tire protrudes - slight tradeoff but much easier to fasten and retain on the car.
Sort of like stuffing a really big tire in the fender so you can't get your hand in there between the tire and the fender opening like the guys say on the xB forum - if you can fit your hand in the wheel well your tires are not big enough!
Well, I'm pretty certain something like this would work, but it would take some ingenuity (and wheel clearance) to get it strong enough and to make it fit. It's the only way I would personally feel comfortable doing it.
Basically it's just a framework that would be attached to the steering control rods so the framework would turn with the wheel. Well, at least it gave me the idea of plexiglass or lexan wheel skirts while I drew it up...