There's no definitive answer to this question. The logical solution, perhaps, is a high-quality (read: expensive) multi-layer treated cloth cover; however, I've had luck with a Tyvek cover from Budge that I've used for years that, on initial inspection feels like it might be too rough but never produced visible scratches in my BJ8's paint over 100K miles of long road trips (we always cover the car at night for security if not protection from the elements). Disclaimer: When I bought the BJ8 it had a fairly new paint job that appeared to be some sort of enamel that has held up well, despite my best efforts to sabotage it. If you have a lacquer paint job, well, YMMV.
Amazon sells what I think is the contemporary equivalent of a cover I bought over 20 years ago at Costo for about 30 bucks. The one I have is a #2, and it's a bit oversized but lends itself to being secured in strong wind--e.g. Victorville, CA--with some short bungie cords wrapped tightly around the knock-offs. It has the added advantage of being compressible to the size of a large watermelon, and the fancy cloth one I bought would have filled the entire boot (not leaving room for 2 spare fuel pumps, a spare water pump, a tackle box full of tools, a sizable box of random spare parts and fasteners and, oh yeah, a full-size spare tire ... just because). I think the #1 would be a good fit, even though Amazon claims it's 'not a fit' for an Austin-Healey 3000 (but they claim the same size is suitable in different materials).
NFI (I WISH I had a boatload of Amazon stock):
https://tinyurl.com/gwhf6hc
Addendum: A less porous material--like Tyvek--may help in dust/sandstorms, but eventually the grit will get to the paint cover or no.