I used this tutorial to piece it together, and fat quarters in my (now substantial) fabric stash. The only modification I made was to make a pouch -- 12" x 16" -- by sewing 3 sides together. Then I quilted the pouch with batting instead of interfacing. This eliminated most of the seams inside. I was going to put bias tape on the corners, but decided against it at the end.
Then I realized the ice pack I carry with my lunch would be more effective if I held the to-be-cooled food NEAR it. So I made another one.
(Please excuse the zipper stitching. I wasn't aiming for beauty on these -- just functionality. There, at least, I succeeded.)