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!