I've been using the ubiquitous blue tarps for 8 winters here in No Berwick, ME. They're cheap, and do the job. I make a frame by lashing 1x3 strapping to put a steep pitch over the cabin. I've never had any mildew problems over the winter. As others have said, I can get access anytime by untying a couple of lines. I typically do get into the boat at least once during layup.
I'll be pulling Hunky Dory this week, too. The bouys are already being pulled around here.
Rick from Maine