I started by drawing the completed bag as it would look as a rigid structure. I did this on engineering paper with the standard top-front-left views. Then I unfolded that in my mind and sketched that out.

Then I drew the the unfolded bag in Adobe Illustrator, and added a half inch of material to make my seems. It was rather large, so I had to tile it onto 8 or so pages of letter size paper. I added registration marks all over so that I could easily line things up when I was taping all the pages together.

Attached are scalled versions of my pattern. I made it like this to get the lagest bag out of the 16x60 piece of ballistic nylon I already had. I cut the coated vinyl in one piece, basically just added two inches to the lower half of that pattern.

Sewing order is something like this:

Sew velcro and reflective strips onto material as appropriate.
Sew side pieces to lower half of nylon.
Sew nylon and vinyl together.
Sew and tape lower edge (completed front and side edge)
Sew and tape verticle (side) seems and cover flap.
Sew and tape bottome seems.
Turn bag and attach strap.