I used a water proof fabric I had from my old job, after we cleared the studio - no idea what it is - would have been used to make snow boarding clothes many years ago. To begin with I quilted the whole fat quarter quite heaverly and then cut out my pieces. The gusset was cut and I inserted the zip before constructing the bag. The lining as handsewn in afterwards, as I found that the quickest and easiest way.
Delighted with how it worked and Ms T loves it.