Any ideas for this bag?

Just got this e-bike for Uber deliveries. I have this monster of a bag for this little tiny space of a back rack attached to this bike.

The bag opens up for the top to grab the food out, so can’t tie it from the top.

Any ideas for how to secure it here and tie it down without reducing mobility?

Measurements are below:

5.5″ x 14″ back rack
14″ x 17″ for the bag

Thanks!

by Brodin-of-Swolehalla