This image shows where I am placing my 2 seams. In the previous posting above this one I mentioned that you don't have to buy but half as much fabric if you use 2 seams instead of 1. I have made my screened openings smaller than the original to increase the strength and stability of the canvas. I like designing in 3D CAD, great tool as you can see it and analyze if that is what your really want. Then I can create 2D measured drawings as well or just have my CAD program open and refer to the model as I layout and fabricate pieces.
There are two long panels each with a long side and a short side and then an extra length goes around the corner so that the seam does not end up in a corner.
The screens overlap the cut line for the zipper by 3/4 inch at each edge.
So the total screen size is 1 1/2" longer and wider than the rectangle you lay out for the zipper cut line.
My CAD model shows using a 5 inch radius for my upper zipper corners. It does need to be a generous radius for the zipper to travel the corners easily. It could be more than 5 inches but I think that is about right for the minimum of the radius. A 10 inch or even a 12 inch dinner plate would make a good template or of course you could draw one with a pencil compass and make a pattern out of cardboard. I will likely use a 6 inch radius as I have a heavy duty zipper and it could use that larger curve. But increasing the radius means I will likely have to add a little filler piece in the placket at those two upper corners to protect the zipper from water infiltration or else as I was already thinking about doing increase the width of the plackets.
When you cut the screens it is important to note when you go to cut screen material that the edge of the screen that will be at the lower end of the panel must have that lower edge placed right against the woven selvedge side edge of the screen material. That edge will not fray which is nice because then you don't have to turn under the edge or worry about those fibers coming unwoven. There is no placket covering along the bottom edge and it is important not to put one there as that allows rain to drain freely off the surface.
It is always important to ask yourself why things were designed in certain ways. In this case the selvedge edge of the screen and lack of placket at the bottom are designed that way to allow water to run off without entering the trailer when the opening is zipped up.
I am not showing the plackets in this posting, I will get to them in a follow up post.