DO a search (make sure the time period specified is at least a year) in RC Universe on "Cessna" and you will find some pictures of the GP Cessna. This should give you some idea of how others have cut their cowls. Ignore Topflite, etc. Generally you should open up the front cowl as much as you can stand and then I would open up the bottom exit area around the nose gear strut. Here is a pict of mine where you can see how I cut the bottom of the cowl. You can see where I put a delta shaped hole for my OS52 valve cover.