I just got a custom-fitted "coverking" for another car, and after owning a number of "generic fits many cars" covers, I'm really amazed at how cool the whole custom-fitted thing is. Rather than having a whole bunch of wrinkles that look awful and pool water and debris, you get a sleek perfectly contour-fitting cover that hugs the shape of the car, and readily beads water straight off since there are virtually zero wrinkles. Based on my research, there are basically two companies (based in the US, anyway) which make custom-fit covers: covercraft, and coverking. Since they build whatever cover you want to order, you can also often specify desired options, like colors (some allow multiple colors), reflective piping, contrast stitching, etc.
I'd be curious to know what manufacturer Porsche uses for their Porsche-branded (obviously, custom-fitted) covers. The one I got, which was officially VW branded (sold through
Volkswagen / Coverking: Home ), was made by coverking.