Quote:
A warmer cozy shape is one that is cylinder shaped instead of envelope shaped. It is enough warmer that you may get by with thinner, lighter insulation.

Envelope shaped works just fine - you want close fitting, too big you end up heating the air in the cozy.
This is an example of the fabric cozy I invented a couple years ago and produce now:


One in use:
_________________________
Freezer Bag Cooking, Trail Cooking, Recipes, Gear and Beyond:
www.trailcooking.com