1
You can either:
a) require all players invited to spring training camps to stand in their traditional batting stance and hand-record the vertical height of the bottom of their knees and the midway point between their belt and their shoulders (possibly flawed if the player crunches his zone down and then adjusts it during the regular season)...
or
b) place shiny target dots on their uniforms at the outside edge of the knees (just below the kneecap) and under the crook of their arms (right about where the top of the zone should be) and triangulate them the same way you triangulate the ball, giving you a dynamic strikezone height that locks in when the pitcher comes set.