1. I took a cardboard sleeve from Starbucks, and opened it up to make a guide. As you can see, I did not follow the shape exactly but it really helped me get the correct curve.
I made the template at least a 1 inch wider than the cardboard sleeve, and 2 inches longer. This compensates for seam allowance.
I made a template for the inner insulation which was about the same size as the cardboard sleeve.