At a Glance
$20 - Costs vary based on price of bag and quality of hardware (if you have a bag already, cost is ~$5 for new hardware)
15.4" Laptop, Charger, Mouse, Fleece Sweater, Brick Digi Camera, Multi tools, cell phone, wallet, city maps, spiral notebook etc. etc.
The bag can accommodate 2L bottles (3) and even gallon (milk container form factor).
Step 1: Choosing your pannier style
Of course, there's your DIY standard "bucket pannier" made from old cat litter buckets (or similar) as seen here or [http://web.archive.org/web/20050208161130/http://members.rogers.com/bphuntley/BikeBucket.html here] or even here. While utilitarian and high capacity, these lack a certain sense of sexiness. You can make the panniers seen in this instructable for the same price as these bucket panniers if you use some recycled material.
There's also the surplus army bag conversion - [http://web.archive.org/web/20041020064156/http://www.twowheelfetish.com/Ezine4/diypanniers.html as seen here by Peter Moore].
You could, gasp!, buy panniers... Arkel makes some great bags - the one closest to my DIY version is the Arkel Bug Pack. It's convertible - it goes from pannier to backpack easily. It also has a quality engineered mounting mechanism. The price isn't something I get excited over, $165... But I've met a happy customer, and will vouch that it is quality.
Photo Credit (Buckets): Brian Huntley
Step 2: Supplies
*2 Mirror Hooks that can hook onto your rack
*4 Machine screws that fit mirror hooks*
*4 Lock washers
*4 Nylon acorn nuts
*1 bungee cord (with at least 1 hook)
*1/8" Plywood (I recommend cheap Lauan aka door skin aka thin 3 ply plywood) OR 1/8" (or 3-4mm) acrylic
For all hardware, cost depends on quality. I went for stainless steel where possible. While I won't be keeping these outdoors, I won't need to worry so much about corrosion.
*I used button heads with a hex key slot. I carry a hex key set with me and there's a suitable amount of engagement to prevent stripping.
Step 3: Things to Look For in a Bag
This is a super thin pocket used to store something like a map. In the case of my bag, it stores the backpack straps when not being used as a backpack. This is very helpful to have as it keeps your backplate separate from the main storage compartment. This pocket should have a zipper to keep it closed.
As you'll probably be mounting and dismounting this pannier, it's a good idea to have a handle to carry your bag with. It serves another purpose by providing a nice sturdy grabbing point to remove and mount the bag.
If the bag is too small, it probably won't be very useful. By the same token, bags that are too big are easily overloaded and don't make for easy travel. Ask yourself what you need - what you really need.
Extra super nice features include buckles, hooks, etc. My bag has two hooks near the bottom where the backpack straps mount - this is a perfect mounting place to secure the bottom of the bag.
You don't want a bag with too many dangley bits. These are liable to get caught in your drive train or spokes unless managed.
Step 4: Backplate
If you absolutely can't fit the backplate in, trim it down.
The backplate is necessary to hold the bag's shape. Without this, the support hooks are free to move making the bags insecure.
If you don't have an inner pocket - follow the same instructions, but don't make the vertical dimension as tight. The difference in manufacturing bags with inner pockets versus without will be start in the following steps.
Step 5: Hooks
I used double sided tape to quickly mock up where my hooks would be placed and make sure my bag would fit properly. Fit was important for me as my seat slings over my rear rack.
Once you've found where you want to place hooks on your bag, slide your back plate in your bag where you would like it to stay. Then mark and cut through your bag material.
For Nylon (and probably other synthetics)
Ladies and gentlemen, start your soldering irons. Cutting the hole with heat is highly recommended to fuse the material together such that it doesn't unravel. It also melts a little bit of plastic (or burns wood) on the backplate to mark where you'll need to drill.
For Natural Fabrics
Cut with a knife of scissors. It's likely a good idea to get some grommets to hold the edges together and keep it from unraveling.
If you don't have the inner pocket - you'll want to add more support. You don't need extra hooks, but add a couple bolts to give extra holding ability. Perhaps one right in between the hooks and one near the bottom of the bag.
The reason for this is that if the backplate is stored in an inner pocket, it's technically resting against the top of this pocket (and perhaps the zipper). If there's nothing to push against it in the main part of your bag, you won't be able to mount your bag without it sliding off.
Step 6: Drilling
If you're adding more support bolts - now'd be a good time to drill out those holes too ;)
Step 7: Fasten your nuts
Step 8: Bungee Time
I cut the end off my bungee cord and then used a match to sinter the ends closed (to keep it from coming apart). I then removed the two hooks. I strung it through the first of my bags plastic loops, added a hook, then through the second plastic loop.
I then mounted the bag on my rack and eyeballed just how much bungee was needed to secure the bag down. The idea for the lower bungee is to hole the bag down - so you'll have to mount to something that is below the bungee, not above (otherwise you'll be pulling the bag off the rack rather than holding it down).
Once I had my length, I knotted the end with a simple overhand knot. The extra bit of bungee was tied up with a bowline knot. Why knots? Because knots are easily adjusted ;)
Step 9: Reap the Benefits
As a note... Yes, I am attaching my bungee to my swing arm. Yes, the distance between the bag and swing arm changes when going over bumps. No, the bag doesn't jump off. I've added a little extra tension to make sure, but the act of the bump will tend to push the bike up and newton's law will tend to keep the bag where it was (at that point, below the rack). No problems thus far riding on the streets (and off the sidewalk curbs) of San Francisco.