Step 2: The Mouthpiece
My mouthpiece parts fit so tightly that no glue was needed.
I cut the 1/2" PVC channel and put it in place to mark the size of the rectangular sounding hole. Then I removed the 1/2" PVC, drilled the hole, and touched it up with an X-acto crafts knife. You need a narrow and sharp blade to get in the hole and carve out the wedge-shaped edge of the hole.
I put the wooden dowel plug right up to the start of the sounding hole. Setting it further back toward the mouth changes the pitch and allows some tuning. I used a metal drift pin and a hammer to set the plug inside the pipe.