I am trying to find some ideas for converting a mess bag gather dust in my garage into a pannier. The bag I am hoping to use is a Timbuk2 large size mess bag. This is big enough to carry everything I need for work plus my lunch.

I have seen a couple ideas for backpacks (there is one really cool thread showing the use of PVS pipe), but nothing in reagards to mess bags.

I wouldn't even care if the bag sat on top of the rack instead of being on the side, but I am kind of at a loss on how to best attach it. It would need to be able to come off the rack...

If you can find some heavy duty cardboard or a piece of coroplast you could use it inside the bag on the side you want to attach to the bike. Then (after measuring where you want to attach hardware), punch small holes through the backing material and bag and rivet hardware like this https://www.jandd.com/detail.asp?PRODUCT_ID=FWSA-HJH and this https://www.jandd.com/detail.asp?PRODUCT_ID=WSA-BUN2.



Good luck.

If you don't want to order online, you can make your own hooks out of framing corners at your local Ace. They have holes pre-drilled but you need to bend them.
You can also make the "fish hooks" out of frame hangers... again bending them with a vise grip.

I've tried, actually the exact thing. Large Timbuk2 messenger bag, and I realized it's simply too big to fit on most racks. It will extend far beyond the rack and dangle into the wheels.

Yeah, after some measuring and testing I figured out the Timbuk2 bag I have simply wont work without a lot of extra internal support work. I ended up creating a pannier out of a backpack I have been using for work and school. It is a little smaller, but the shape lends itself to being converted better. I basically followed the idea behind this tutorial and adapted the design to fit my needs.