Dick is right on. I don't think you need another exit hole at the top. Just add a fence or dam at the forward edge of the exit hole on the bottom of the cowl to encourage low pressure behind it. That will cause air from inside the cowl to "fill" that low pressure, and more air will flow through the cowl. I have done this several times to improve cooling, without altering hole size at all. I've seen it on some full-scale airplanes too. A small fence (even only 1/4" or 1/2" tall) should make a significant difference in flow rate. No calculations necessary.