The replies are right on. At least 1 : 4 inlet / outlet or more is the strongly suggested norms.

I think you might be concerned about the lack of stiffness or strength in the remaining cowl ... so what I do is cut out plastic or metal screen material (like door or window screen), fit it into place in the bottom of the cowl then lightly epoxy the edges of the screen to the cowl. Gives the cowl perfect strength, and it's "light and airy" for the engine cooling. I leave about an 1/8" space around the exhaust stacks and also finish the edges with light epoxy, too. Works great.