Hi George, I have no info on making your own, but if you are talking about the 18" WSM, here are the approximate dimensions of the ring:
Diameter: 14 1/4 to 14 3/8 inches
Height: 4 3/4 inches
Air Holes: 36 holes; 2 rows of 18 holes
Hole Diameter: 1 1/4 inches
Edge of each hole is 1/2 inch...