Clips for attaching plastic box to bike rack

I don't know if this will be of any use to anyone else because it is so specific, but I thought it was an elegant solution and a perfect use for my Makerbot.
My friend asked me for suggestions on a wobbly installation he had done.
His daughter wanted a plastic storage box attached to the back rack of her vintage bike. As a college student in theater, she carries lots of materials back and forth on her bike, and a girlish basket she had just did not cut it.
My friend had attached the box wi