I would also ask why remove canvas. No one tells you to do that.
The tarp thing has been discussed forever. You can use a tarp if you don't wrap the entire unit up and have the tarp hang down the sides. I've always put 2x4's on the roof to keep the tarp from layering the roof. And you simply have to tie down the tarp so it doesnt flap around in the wind. As another poster mentioned, you can wear the finish if the wind moves the tarp around. No rocket science here --- just don't totally wrap the pup and secure the tarp.