Steve, I made mine tonight and only have a few hand stitches to do but so far this is a great project. I changed one thing:
While I bought 2 yards of ripstop, I bought 2.5 fleece. What I did with the extra half yard was to fold it back over at the foot end and sew it. The seam I put on the inside between the walls. This way I wouldn't pull it loose when putting my feet in. It allows my feet to have a 1 1/2 foot of double fleece.
I'm glad I found your thread. Thanks!