It looks like you have made an exact frame to make precise templates for your panels. But a word of warning - if you plan to use silnylon fabric, it has some stretch. If you cut the fabric to the same size as your templates, you will find that the bottom edge of the finished flysheet will lack tension.
If I were doing this again, I would cut the silnylon panels to the template pattern, then sew them together with a single line of stitching. Then place the flysheet over the frame and check the tension. I would expect to have to adjust the lower part of the seams to increase tension in this area. When adjusted satisfactorily, complete the flat felled seams with the 2nd row of stitching.
However, if you are using cuben fabric, this has no stretch so you will not have this problem.