Bertelsbeck has them - way cheaper than the Alfissimo guy, but after shipping maybe not: Alfa Romeo SPIDER (105/115) CAR COVERThat's a Centerline offered cover? I didn't think any of those were that nice looking, with the side view mirror cutouts! Cool Alfa logo, also.
I like what the prev. poster said about fast temp changes. I chased condensation issues in the trunk of a BMW for years; it would form like on a tupperware lid and wet the carpet in the trunk, and form on top, also. Even with a car cover on. This was on cold nights. Maybe crack a window to allow the interior to equalize with outside the car?