Why in the H didn't I do this when I started to glue up the 1st seat, instead of only remembering to do it for the last seat.

I attached 2 scrap blocks to my work bench at perfect 90 degree angle. Then as I assembled the seat slats, and frame, I just pushed the whole assembly in to this corner hard, and it squared the seat, kept it flat and let me make minor adjustments to the alignment of the corner. I left the gap between the two pieces in the upper left so I could see and get to that corner as well.