The covers for the 18.5s are a little tight especially on the newer model. I have two handles on my newer 18.5. I did have four handles but took the other two off because of the cover. I also cut the elastic tabs off the bottom of the cover where they tuck over the vents. It slides over the two handles and stays secure.
I bought a used 2008 WSM. The cover that came with it was really snug, but the cover I bought to replace it was much roomier. It fits over the four garage door handles pretty easily.