Thank you, Tartan, for posting this. I really like that it can lay flat.
I used to make 'controller bags' along the same idea, but kind of inside out from yours so kind of boxy with the ends seamed and on the inside. The one I did with lining started out about 12 1/2" x 17". I did a few with lapped over opening and then sewed Velcro to keep it closed.
I need to make some more, so will try this way so they can lay flat. I have lots of zippers, but most are shorter, so may have to make smaller pouches.
Janey - Neat people never make the exciting discoveries I do.