As Sophie has increased in age, her collection of duplo has also increased such that it finally exceeded the box we had it in. Thankfully, the internet is full of wonderful ideas about what to do in such a situation, and I had stumbled on this idea recently, so I looked through the stash and did it.
The outer fabric was left over from the back of Rachel's Quilt. The inside is from who knows where. I did buy the cord at 90 cents a metre from Spotlight. All up, a $3.00 play bag. I call that winning.
It's only about a metre around, but it fits all of the duplo very comfortably and there is room for growth too, so it will do suitably for now. Hooray!