Grwoolf:
I haven't read all the posts here because your thread got long so I apologize if this is redundant. If I were you and couldn't put a structure up at all, I'd do the following:
Find a manufacturer of the newer "breathable" car covers like those I use for my Porsche's from Tweeks, Mid America Motor Works...etc. They protect against UV, are waterproof, breathe to allow moisture to escape and are designed to allow a very expensive automobile to exist outside in the elements. I'd simply work hard to find who manufactures this material and try to buy a 20 by 20 foot or 12 by 12 foot or whatever piece. Then I'd have the sides sewn, or have them sew them so they don't fray, and put grommets in myself with a $10 kit from Lowes. I'd park it, tie it tight, put stones on the edges, whatever. I'd think for a few hundred bucks you'd have a breathable durable cover that would last at least a couple years and prevent moisture buildup. Think how lucky you are compared to us up here in the Northeast where we might have three feet of snow sitting on the machine.
Although this is far from ideal it might be a solution. But I'm assuming a portable "car port" like from Cabelas is out of the question too according to your post? That would be my choice if you could do it.