The bag that Dunster has is similar to the bag in the book ABCs of LongarmQuilting by Patricia Barry. This is the one I made when I first got my longarm. I used my practice sandwich to make it. I added a pocket front and back and when I added the extra handles - I gave the webbing one twist so when tucked into the pocket it would lay flat.
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My only mistake was not making the bottom flat (it looks like Dunsters sit flat on the ground) but it was easy to make and I use it all the time.