Step 5: How does it stay on a bike?
Take four, two inch longs strips of Velcro hooks and place two on the front and two on the back of the lower part as shown in the photo below.
Then take four, six inch long strips of Velcro loops and attack them to the top part of seen in the second photo but make sure there is some slack of them to open and close / hook onto a bike.
Voilà. You now have a saddle bag. Not only have you recycled but now possess one of the rare and coveted saddles bags made of milk bottles that will be the envy of all your friends.