First and most of all, get a temperature gauge or gun. There will be no way to determine what might be needed without a way to find the temperatures of both heads. The exposed singles don't need any airflow help, so leave them alone.

Air will, like water, normally take the path of least resistance. If it has the opportunity to pass relatively free and unobstructed over, under, or around an engine rather than pass through those restrictive cylinder fins, it will do so. Some cowl designs are better than others in positioning the cylinders in the direct path of cooling air. Some engines may sit too high or too low to benefit from the incoming air. This is where the engine temps come into play.

Get the temperatures of each head with the engine fully exposed, running at a reasonably high, and preferably max rpm. Try to get your measurements from as close to the spark plug as possible. This will most often be the hottest location on the head, and one head will usually be hotter than the other one. Use these temps as a baseline.

Install the cowl and repeat the temp checks. If the temperatures are hotter, and they probably will be, you need a baffle. The baffle will need to be installed in a manner that directs the air through the cylinders and out the back, not just to the cylinders. You may need to build a baffle that completely prevents the air from passing below or around the tops of the cylinders. You may also need to build a "plenum" to properly direct the air through the jugs and out the bottom of the cowl.

Each plane/engine combination is different, so there's not much anyone can do about specific measurements. Good pics can be found at several sites. Brison R/C Engines in the 6.4 engine section is one. The Wild Hare Forum in one of the G-202 threads is another, and the other engine manfacturers site already noted.

Some will jump on this one, but you want to keep the temps at all times from exceeding 325 degrees farenheit. That's even pushing it a little. Engine melt down will occur at somewhere very near 400 degrees f. It won't hurt a darn thing to rum at 300, but do try to stay under 300.