My neighbor may be building a fence soon (my deuce is too wide for my driveway and is partially on their land) so I've made arrangements to store it at a friend's place, out in the woods. I've already looked on Amazon and found a
20' x 40' tarp that I was thinking I'd stake around the truck to keep the worst of the weather off it. I hadn't thought about condensation mentioned above, but in that case maybe just keep the windows/windshield cracked (and screened) to allow air flow.
I haven't tried the above tarp yet, but it looks promising. Alternatively, you could go for something made out of
canvas. I bought a smaller one of these, 6 x 20 to use when crawling around under the truck. I don't know how well they'll do in the sun, which is usually a killer on fabrics. And it's also 4x more expensive than the one I list above. The 20' x 30' is $130 cheaper than the 20' x 40', depending on how much coverage you want.
Keep in mind the finish size and stated size are often off by a little bit.