Make sure you leave enough room in the door opening to allow for the canvas and paint.
I tried a different approach. It was a bit of a risk but I thought it was worthwhile to try it. Time will tell if it holds up to the elements. As I glued the canvas on the outer wall, I covered the opening with the canvas, then painted on some thinned glue where I needed to cut, then I let it dry and I cut it with a very sharp blade. I cut as close to the edge of the opening as possible. I then used the titebond 2 wood glue to really glue down the edge of the canvas if it wasn't already stuck down. After it is painted it looks like this.
I did this for all window and door openings. I'll keep everyone posted on how it holds up.
Also, I didn't do anything for a door threshold, and am thinking about cutting the bottom of the door, or at least shaving it.
I made my threshold part of my door frame. It is hard to see since it is all painted white. I placed a small piece of scrap wood where the door sits when closed so that the threshold shows up in the picture. I tried to leave enough room for a seal so that the door will smash it when closed. The threshold is just a little more than 1/2 inch tall and it goes all the way around the door frame.