The first version I tried was just a tote bag that I strapped to the rack. I kept adding pieces to make it easier to attach, like velcro and buckles, it looked pretty funky but it did the job. I looked up some options on bags to buy, but they seemed more geared to hardcore bicyclists and tour cycling. I just wanted a bag that I could pop on and off my bike. I also didn't need two full saddle bags and I liked the feel of something that I could attach to the top of the rack.
Last week, the zipper broke on my funky bag, so it was time to tackle making a custom bag with my personal specifications. The biggest factor was making it attach securely to the rack, while still being easy to remove. Every week I take a different amount of things with me, so I wanted a way to make it expandable to fit a variety of things.
I do have a fair amount of fabric in my stock so when I begin a new project I try to use some existing instead of buying all new materials. A friend's mom had given me this upholstery fabric a couple years ago for helping her get used to her sewing machine. It's feels like embossed vinyl with a fabric backing, it probably has a real name that I don't know.
There is a zipper pocket in the front for easy access to small items. There is a removable cardboard panel that slides into the bottom interior to give it some stability so it doesn't just flop over the rack.
The panels on the bottom wrap around the sides of the rack and snap to the back, it's actually really secure but still easy to get off again.