I just made something similar for my daughter last night. It looks like a Japanese Obi, sorta. All you do is make a big pocket, I think it was about 10x6 inches with wide ties attached. You could make the pocket part any size you wanted. You could also make the ties as wide and long as you want to as well. You attach the ties to the pocket before you turn it. The pocket is made similar to a pillow cover, where the opening in lapped, but there is no button or zipper.
The warming insert is made just slightly smaller then the pocket. It is simply a square, or whatever matches your pocket with tunnels sewn in to keep the rice or flax separate and flat. I made three for my daughter so that she could warm one and freeze one and have a spare.
The only thing I would differently is probably make the ties a little longer. I think ideally they should be long enough to cross in the front and then go around your waist and then tie in front.
I made this for her lower back, but it could be used anywhere you needed hot or cold therapy. Plus you could actually use the pack part for keeping food warm or cold as well.
I found the pattern in the One Yard Wonders book. But I did a little adapting.