Lots of good info here and it seems the right size in the right place is the key. However, form all the pictures, there is more then one way to get the job done. Here is a picture of my latest attempt and it may get mods depending on what Andy lennon's cowl chapter says or what you guys say.

I used contest balsa and wrapped it around the front half of the cyl. There is a medium size opening in the front with a height of the cooling fins. The cyl head is partially exposed and the tapered outlet to the rear is at least 1.4 times the fin area. This cowl design when finished will be tested on a inexpensive engine before going on a speed engine.