Most of the stuff online for pannier conversions for your regular bags either make your bag about as uncomfortable and unwieldily as the ones in the store, or you have to sew in or rivet hooks and straps that the bag isn't designed for. So you'll end up having a ripped bag or re-inforcing it.
So, here’s a bike pannier conversion you can make for $10 (or so), if you have access to some basic tools. It requires no modifications to your bag. All you need is a strong place to mount from, i.e. steel rings, heavy duty nylon, even your backpack straps. Anywhere the bag is already designed to be carried from and to take the load and beating over the lifetime of your bag. My Timbuk2 bag has nice steel rings, where the shoulder straps connect to, and are the perfect place to mount a bag to your bike.
Things you will need:
- (2ft) 2×2 pressure-treated/outdoor wood, a.k.a. deck railing. Found at any home store.
- (2x) Threaded eye bolts for wood
- (2x) 2.5″ J-bolts with washers and NYLON locking nuts (At my local Tru-value. Could use U-bolts too.)
- (2x) Cheap aluminum carabiners ( Or go more expensive and get some Nite-Ize S-Biners )
- Optional: (2x) Eye bolts and zip-ties
Measure the length of your bag from mounting point to mounting point. For me, this is about 20″ from steel ring to steel ring on my Timbuk2. Two eye bolts will be placed at each end of the 2×2, so subtract about 2″ to compensate for them. Then, cut the 2×2 wood to length (18″ for me).
Shape the 2×2 so that it sits flush on your bike rack. The wood can be somewhat soft. I used a handsaw to get most of the material off and use a screwdriver like a chisel to get the rest. Don’t go nuts on this and leave as much material as you can. Just try to make sure that you don’t make sharp corners or weak points, because you wouldn’t want your bag to fall off your bike, if the wood breaks.
Drill pilot holes for your eye bolts at each end of the 2×2. Screw them in so they are snug but not going to split the wood.
The J-hooks secures the 2×2 and your bag to your bike rack by hooking the railing of your bike rack through the 2×2. The nylon nut keeps it tight and ensure that it won’t loosen over time. Locate a couple of good places to drill through-holes for your J-hooks and bolt down the 2×2 with a washer and locking nylon nut. Get it snug and tight, so the 2×2 is fixed to the bike rack. If you’re so inclined, hacksaw off the excess thread on the J-hooks.
If you have a U-lock, I added a place to mount my bike lock, which also keeps my bag from swinging into the rear wheel. Otherwise, you’ll probably have to attach some kind of stand-off to your rack, so your bag doesn’t get sucked in and destroy your wheel or bag. For added peace of mind, you can zip-tie the 2×2 to your bike rack, but if you did it right, you shouldn’t have to.
And don't forget to tuck the loose straps!