If you don't have an Alfa Accessories Catalog, the OEM luggage rack is still fairly easy to recognize, as it is about the only one with 5 rods. Below is a picture of one. It is a permanent mount type, and Alfa never offered a removable rack - at least not for the traditional Spider. The OEM rack is sturdy, and distributes the weight very well. You would be surprised at how much weight it can carry without damaging the trunk lid. (When I was in college, on more than one occasion I moved my solid wood desk by placing it on the rack. It looked much bigger than the car!
By the way, it was a nice desk, and I still have it!
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There is one company in the U.S. that offers very nice removable racks for Porsches, Miatas, and Hondas, but not for our classic Alfas. There are European outfits nonetheless, which IIRC do offer removable racks for our Spiders. I'm more certain about there being one for the series 4 Spiders, than the other earlier models. I don't think that one was ever made for the Spiders with the rear spoiler, since most designs secure the removable rack by wrapping around the front and back of the trunk lid. If I were looking for one, I would start by searching in the German web sites.
Best regards,