By the time I got to this point, I realized that I would need some way to join the sides once I folded them up. So I made some provisions for flaps, to be folded on the inside of the corners where the sides meet.

In this photo, I have cut into one of the corners by 2" and a hair more (the 2nd vertical line represents the 2" mark). I have also cut off 1.25" of the corner area, leaving a 3/4" flap.