I'm a new WSM owner, and all these ideas sound good. I have had a Weber kettle for a few years, and I have the cheapie gray cover. I leave my top & bottom vent open when stored under the cover and I have not had a problem with moisture at all under our covered patio. When I went to pick up my WSM last night, I removed the black vinyl cover & found some water inside the charcoal bowl. Since the black vinyl cover is is listed as a premium cover, I assumed, incorrectly, that it was waterproof, or at least water resistant. I held up the cover to the patio light, & saw some pin holes & light coming in through the seams. Saw where Chris A. said the cover is not properly designed which explained the water, so I will give the 55 gal. bag + cover idea a try for my WSM and maybe the upside down center section idea too. I plan to leave all vents open as well.
Thanks for the tips.