So, this may not be of precise help, but I just measured my side tables / wing for a 23". They are, of course, comma-shaped with a radius which hugs the body of the grill. For a 23, the widest part (from the body radius perpendicular to the edge) is 14.5". The depth, front to back (the length of the "comma", but measured as a straight line), is 21".
For comparison, on the 32, the Side View Outside dimension (above chart, right view) is 35". On the 23, the same dimension is 31", which fits the 21" deep side table. Some proportional math should get you in the ballpark for your square of plywood. Hope this helps.
Jon