Well mother nature just posed an interesting question... What to do when it rains?
Answer: Summon your inner MacGyver
I removed the kettle-end top panel from the cover and then just backed the stitching down a couple inches on the table side until there was enough room to let it slide down over the WSM:
Not going to win any beauty contests but it should get the job done.
I may actually cut out the top properly and stitch the WSM cover directly to the Performer cover.