The first cover I bought was a single layer and I found it leaked and trapped water in between, (dirty water) so it was pretty pointless in keeping it covered in the first place.
The wind ripped it off and it was binned.
The double layered has stayed waterproof after 9 months of use, I tied loops into the material to tie it to all four wheels otherwise it will blow away in heavy wind. The other problem i found was on the gt4 after a good run i have to leave it to cool down if i put the cover on otherwise the heat from the bonnet vent will melt the fabric.
Mine is just a large cover not model specific and it fits fine when i fold the mirrors in, other thing to watch for is if you have an alarm with proxy sensors it can set them off, so I just disable mine when the covers on.