I put a Topeak rear rack on my commuter a couple of months ago. I soon realized that putting my gear in a bag and cinching it onto the rack didn't work very well. I thought about getting one of the Topeak trunk bags made to fit onto the rack, but one was too small and the other had zip down panniers, which I didn't need, plus it cost $90.00 plus. I decided that I might be able to make something that would work well for what I needed.

After a lot of looking around, I went to one of the box stores and bought a mini-cooler for $14.99. Measured it in the store to make sure it would fit on the bike, correct height, be big enough for what I carry, etc. A couple of things I wanted were a carry handle and a top or side pocket for loose stuff--sunglasses case, repair kit, etc. The cooler I bought had those things.

I needed a way to attach it to the rack, so the first thing I did was cut and shape a piece of plywood which I attached to the bottom of the bag using screws. That lasted until the bag took a hard knock and the plywood shredded and came off the bag. Back to the shop.

I found a piece of solid wood in my garage and with the help of a friend who teaches wood shop, shaped it to fit into the Topeak rack (cuts on the corners at 45-degrees--see picture). Then I drilled through the bottom of the cooler and attached the wood using #6 x 3/4-inch screws and washers (#8 screws would work well too). Found I had to make the bottom stiffer because the bag was moving from side to side as I rode, so I cut off the front cover of a discarded notebook and used that as the base panel. Works great!

You can see pictures of the completed cooler commuter bag below. A few specs:
Bag: $16.22 plus .70 for hardware. Total: $16.92. Wood, had on hand (even a piece of wood from an old pallet would probably work, but you'd need to plane it down), plus notebook cover (free).
Total weight: 1.45 lbs (Topeak EXP is 2 lbs 13 oz.)
Pleasant surprises: Keeps my lunch cool, has a reflective zipper, and it's padded!

I sanded the edges a bit to get the size right, but rubbing the edges with soap or wax makes it slide on and off easily. I don't have any trouble with it coming loose, but if I did I'd attach some kind of clip between the bag and seat post (the original cooler had a shoulder strap that I cut, forming a pull loop on the back of the bag).

That's it! Worked for me. It may work for you too!

Nice job! I've thought of doing something similar, but I've got a sewing machine that's tough enough to attach some straps to fix it to the rack. Just need to find the right bag.

Now that nice. good job.

Thanks for the props, guys!

Not bad, and you could carry a 6-pack in that.

Cool bag... scary seatpost!

Never thought to self-build against the Topeak quick-track system. I've got a tablesaw at home....

I'm thinking this weekend that an milk crate might become a quick-mount!

Cool - literally!

I always wanted to try it. But never figured out how. thanks!

I've used a cooler too. This one gave me space for two more water bottles.

Sweet!

That seatpost (it's a Salsa) goes forever. Can't put the seat all the way down 'cause the post runs into the bottle mount screws. That's okay by me.

Very Nice! Nice way to save a BUNCH of dough!

Nice work, I also have a cooler as pannier. Enough room for change clothes and laptop.

Haha I thought about using my Waitrose Cooler only yesterday I just didnt know how best to make the bracket! thanks for the pics, I will be attempting this tonight!

Nice job

Great job. I love DIY. Submit it to bikehack.com.

I used one one summer, it was awesome but the zipper in the side pocket failed and so I stopped using it. I liked it because it was large, square and had a plastic box inside it that kept it's shape. no pictures of it, I think I got it in the fishing/camping dept. of Kmart. I had to use bungee cords though cuz I wasn't as clever as you! good job!

commuters have the most fun rigging their rides!

I eventually found a bike specific bag that was similar and also collapsed smaller when necessary. It's not cheap and gets selective use here and there. If I start commuting again this will be my bag for that. the timbuk2 hunchback is also insulated