I used 3/16" which is 4 1/2 mm. You probably can get away with 3mm (1/8"). You can glass both sides with 2oz and that should be plenty. The glassed 3/16" part weighed about 4 grams as I recall so weight is really very minimal consideration. If you build the adjusto-stab, you might consider bringing the horizontal stiffener all the way to the post.

MattK