I bought a 12x20 Cover-It Shelter a few years ago on the recommendation of a friend. It would be adequate to enclose your Caravel
without the sides flapping against the aluminum. I am considering buying a second one or a larger unit for my Caravel
The Cover-Its are quite a bit sturdier than the Costco units. They use a pipe frame that bolts together in about an hour or two. I assembled mine completely (frame, fabric, and anchors) without assistance in about four hours.
The Cover-It easily withstands the heavy (12-14") snows we get here, and they claim it can take wind up to 70 MPH. The most wind we get here is about 40 MPH and I can say that it seems to deal with that readily enough.
Two things to watch out for: (1) the ground anchors must be very tight. Consider carefully your soil and talk to the Cover-It people about the type of anchor that would be best for you. If it comes loose, the entire structure can be eventually damaged as the "foundation" begins to move. You can mount to asphalt or concrete instead of soil if you want.
(2) Be sure you are on level ground. I didn't level the ground beneath my structure sufficiently and as a result the structure began to "spread" as the lower side crept downhill. This caused stress on the door and eventually resulted in the door failing ($100 to replace). The stress also caused some of the sidewall connections to loosen up, which allowed the sides and roof to flap in heavy wind, which resulted in pinholes forming after three years. None of this would have happened if I'd had it properly levelled in the first place. My friend's identical unit, installed on level ground, is still in near-new condition despite being two years older.
My father has a Costco shelter that cost him $175 (versus the $700 I spent). The Costco is adequate as long as it is not expected to survive heavy snow or wind. The framing is very minimal and really not designed to take much abuse. Despite the self-induced problems I've had, I think the Cover-It is a much better choice for something as valuable as your vintage Caravel.