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
Step 2: Cut and Drill the Board
1. Cut the plywood so that its width fits snugly inside your bag and the height is a little taller than the bag. My plywood is 12 x 14inches.
2. For ¼ bolts, use a 9/32 drill bit if you have one. A ¼ bit will make holes that are a little small for the bolts. Drill the bottom two holes about half an inch from the corners of the wood (My holes are a little closer to the edges than this, but I think half an inch is better because the wood is less likely to crack.
3. Before drilling the top two holes, which will hold the hooks, take a look at the rack on the bike where the bag will hand. You want the spacing between the hooks to fit on your rack. Rear mounted racks have cross bars that get in the way of the hooks, so choose a spacing that is neither too small or too big. In my case, I drilled the holes for my previous bike and they don't fit so well on my current bike (see photo below). inches below the top of the board.
4. When you have the spacing between the hooks figured out, drill the top hole for each pair about 2½ inches from the top of the board.
Step 3: Assemble the Bottom Corner Bolts
For each of the six holes in the board, you will have to poke a corresponding hole in the canvas bag. I do this one at a time, starting with the bottom corner holes.
1. Insert the board into the bag and find what will be its final position in one of the bottom inside corners. In this position, the corner of the board should fit snugly in a bottom corner of the bag. Holding the board and the bag together, with the fabric stretched, take an awl make a hole into the canvas.
2. Widen the hole a bit by sticking a larger diameter tool in the hole, such as a phillips screwdriver.
3. Work one of the bolts into the hole from the outside of the bag, with a single washer on the bolt (The washer here is important because it helps protect the fabric of the bag from wear). It's a little tricky to get the threads started, but once you have the hole wide enough, you can screw the bolt through the fabric.
4. Insert the board into the bag, fit the bolt through its corner hole and add the nut and washer on the inside of the board and tighten down the nut. Since it's a lock nut, you will need two wrenches to do this.
5. Repeat steps 1-4 for the other corner bolt.
Step 4: Assmble the Hooking Hardware
The photo below shows the arrangement of the hardware.
1. One side of each angle bracket must be shortened, otherwise it would interfere with the hooks. With a hacksaw, remove part of one side of the brackets so there is only one hole and that side is reduced to a length of ¾ inches. Use a file to make the sawed edge smooth.
2. I can't remember if this step is needed or not, but the holes on the angle brackets may have been too small for 1/4 inch bolts to fit through them. If this is the case, you'll need to drill out three of these holes on each bracket with a large enough bit.
3. Sandwich the threaded hook between two nuts and two lock washers, as shown in the photo below and tighten. You could also use nylon lock nuts instead of lock washers.
Step 5: Assemble the Top Bolts and Hooks
1. Pull the top edge of the bag taught against the board, keeping the board centered in the bag. While keeping the fabric taught, poke one of the holes through the board.
2. Insert a bolt from the inside of the bag, passing through the board before entering the fabric. Note that the top bolt uses two washers, while the bottom only needs one. This is to keep the screw from interfering with one of the nuts that holds the hook to the bracket.
3. As before, work the bolt through the fabric.
4. On the outside of the bag, insert the bolt appropriate hole of the right angle bracket and tighten the nut. Be careful to keep the fabric straight and un-deformed while tightening the nut.
5. Repeat steps 2-4 for the other bolt and nut.
6. Repeat steps 2-5 for the other hook.
You are almost finished!
Step 6: Add the Bungee Cord
If you aren't familiar with bicycle panniers, the bungee cord hooks around a hook at the bottom of the rack on the bike. Then, when you set the bag hooks on the top rail of the bike, the cord is kept in tension. This creates a force that keeps the top hooks pulled against the rack. If you didn't have this stretchy cord, the bag would fall off your bike whenever you rode over a large bump.
Originally, I used old bicycle innertubes, which is more eco-friendly, but they wore out fairly quickly and also didn't have as nice of a stretching behavior as the bungee cord.
The bungee cord hangs over the two hooks. Sometimes it comes off, so it would be a good idea to bend their end hooks in so they are captured around the brackets, but I haven't bothered to do that yet.
You are done!