I wanted it set up so that the bag could easily be removed and carried around as a backpack when I parked my bike.
I was inspired by this website:
I was originally going to use the same method of making an internal dowel frame to keep the backpack rigid. But my backpack luckily had 8 metal rings on the sides that allowed me to attach it to an external frame and keep the backpack intact. No holes or sewing required, which is good, because I'm sure I would have screwed up the bag in some way.
My pack has "Bag Ammunition M2A1 034951" printed on it. Unless you've got the same type of backpack, you will probably have to attach it to your bike in some other way. Plenty of good advice here: