This pattern generator details the 2D fabric panel sized to achieve desired 3D dimensions of a round bottom stuff sack with drawstring closure. Referencing the image, provide the desired dimensions for finished size stuff sack.
Channel Allowance of 1.5" (4 cm) is provided.
See drawstring details below.
Lighter weight materials work best for drawstring channels. Here are a few recommendations.
You should now have two fabric panels, a circle and a body.
Your channel should look like the illustration, quite loose around the drawstring.
Congrats, you're passed the crux. Circles take a lot of patience and practice.
Don't sweat it, sewing round bottom stuff sacks is tougher than it appears!