I have been using these bags on my bike for around 15 years. I basically copied the design from commercial panniers back when I couldn't afford to buy one. I take a canvas bag, about the size of a grocery bag, and bolt a piece of plywood inside of it. The two top bolts connect to hooks that hook onto a rear-mounted bike rack.
It makes for a nice, big saddle bag that holds lots of stuff and it's also convenient to carry by the handles when you leave your bike.
Every few years the canvass bag wears out and I replace it. These bags were very common ten or fifteen years ago, but I don't see many these days. Some vendors have sturdy nylon bags that might work as well The photos in this instructable were taken as I did my latest refurbishment.
Step 1: Tools and Materials
Crescent wrench and/or box wrenches
large canvas bag (or any other suitable fabric)
1/4-inch plywood approx 12 x 14 inches (size depends on your bag)
Large Canvass or Nylon Shopping Bag
24 inch long bungee cord
2 1¼ steel right angle brackets with two screw holes on each leg of the bracket
2 ¼-20 threaded steel hooks
6 ¼-20 x ¾" long bolts
6 ¼-20 nylon lock nuts
4 ordinary ¼-20 nuts
10 ¼-inch washers
2 ¼-inch lock washers