I find that tarps do quite well - shorter life-span than a canvas cover, but I have to park under trees, and keeping any cover clean for multiple seasons would be challenge. I tend to get about two-three years out of a tarp and use the older ones on top of the more recent ones to limit discoloration and to avoid rain getting in through weak spots.
Back to your project: there's no reason why you couldn't tent using a canvas tarp (or a custom fitted canvas cover) using the mast as support. (I've since added a third support at the bow, but never noticed any ill effects of not supporting the mast there, earlier).
I think of the big opening at the back as a feature (ventilation) and have never observed any evidence of critters getting in.