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:
Step 1: The Backpack
Step 2: The Frame
The aluminum was a vertical support from a screen door. It had four spots where it could easily be bent to form five even segments. With one side overlapping, it made a square with sides of about 14 inches. The backpack is also a square with 14 inch sides. How convenient.
Step 3: Strengthening the Frame
This held the square together, but it needed more to keep the corners at 90 degree angles. I cut two corners off of a steel plate with a jigsaw to make two right triangles. I drilled holes through the steel triangles and opposite corners of aluminum, and pop riveted those together. Now the frame was much stiffer.
Step 4: Attaching the Frame to the Bike
At this point, I spray painted the frame black to make it look way cooler on my black bike.
At the top of the rack, I used two hose clamps to hold it on.
At the bottom, I drilled a hole through the aluminum and used the same screw that goes through the bike rack and into the bike frame to hold it all together. The original screw was too short so I found a longer replacement.
Although the frame lined up nicely with the rack, I had to move it a couple inches further back on the bike because my foot would hit the backpack when pedaling. This just meant drilling a new hole on the bottom.
Step 5: Attaching the Backpack to the Frame
Instead I found four little flat spring loaded clips that worked perfectly. They were attached to some unrelated straps I had lying around. I sawed them off and pop riveted them to the aluminum frame so that they were aligned with the four pairs of rings on the backpack.
Step 6: Done!
Thanks for reading!