As you saw (and I'm sure as you all remember so clearly!) in the intro video to this project where we looked at the contents of the giant box that everything came in, World Models includes a clear cowl that can be used to plan and perfect cowl openings before committing to cuts to the actual pre-painted cowl. It's a nice feature but since my engine pretty much fit entirely inside the cowl I only needed to cut a few penetrations and it was a lot easier to just use a method that I'm sure you guys also use which is to overlay cardboard like in the following photos.
By first removing the mufflers, plug wires and spark plugs I was able to get the cowl into position and was able to locate the holes for the four screws that will hold the cowl in place.
Next you remove the cowl and attach a strip of cardboard (I use cereal box cardboard partly because it's the perfect thickness and stiffness and partly because I LOVE CEREAL!) over the area where the hole will be cut. Attach the cardboard so that it can be folded back when the cowl is put into place. In the picture below, the cardboard had already been used on the other side so it has a few marks that don't make sense. The red dots though are supposed to mark the center of the spark plug hole even though the angle of the photo makes it look way off.
Next you reattach the cowl and lay the cardboard into place so the marks can be transferred to the cowl.
Now you just complete the layout of the opening and use your regular selection of tools to cut it out. I've been using a step drill bit to cut larger holes in fiberglass and clear plastic, which has been working well for me.
The pre-painted cowl had a few chips and I used a red Sharpie to color in the white spots.
Here are the same steps for the muffler cut out. I kept it kinda small because the sides of the cowl are open on the sides along the fuselage. I may still do custom mufflers for this plane but we'll see about that after the first few flights. I also minimized how much I cut for the exhaust outlets because I may need to cut more for custom mufflers if I decide to build them.
Begin by making a few important marks
and then add to them to make a pattern you can cut out and then transfer onto the actual cowl
The opening for the muffler pipes wasn't perfect at first but it was close enough and only needed to be expanded a little front and back.
I ran a nyrod with a quick link to the choke arm and set a grommet in the cowl to keep the nyrod from buzzing