Traditional windows this size would use a nominal 2x2 muntin but with a profile done on a shaper. They would not be dadoed and built around the plexi becasue that way you can't replace the panes; they use a rabbet and stop or putty instead. The vertical of the cross gets a 90 degree notch both sides, as does each inner edge of the outer frame where it meets the ends of the "cross." The ends of the cross are pointed 45 degrees each side to register in the four notches. It's a very strong construction, simple on a tablesaw.