This was demonstrated in a live chat, this time it's without the chat!
Materials:
2 x 8" [20cm] squares of outer fabric
2 x 8" [20cm] lining fabric
2 x 8" [20cm] squares fusible fleece [H630]
4 x 2" [5cm] pieces of outer fabric for the zip tabs
3" x 10" [7.5cm x 25cm] strip of fabric for the strap
10 zip [or longer!]