I bought an outdoor cover a year ago via Porsche Club GB and it’s fine for me. However, it seems they changed supplier and are selling a different make and more fitted cover now. Mine is called Moltex and has a nice fleecey lining. Bit of a faff to get on though but it’s well made
I would definitely go for one with a fleece liner.
I have a Specialised cover and it doesn't keep dust out, plus it leaves light sort of water-marks on the lacquer (washes off, but defeats the object a bit!) after a while. It's not even out in the open (open-fronted barn)!