Thickness of a well shaped lid varies along the length. Somewhere around 5/16" give or take a bit seems typical at the thickest portion. It tapers quite a bit in thickness from the base to the tip. Too thick doesn't look good, but neither does too thin in my view. I see a lot of contemporary box lids that don't have a pleasing shape. Many look too big and don't have enough taper. Some are way too thick and others seem big and flat. Study original guns having lids believed to be original to the gun.
