I refuse to install a bike rack, of course.
One deadly simple solution: Shanky 2.0 - a shank-shaped (really?!) device that allows to attach stuff to the handlebar of your bike.
All you need: some pvc scrap, a piece of (smaller) metal pipe, pvc cement, zippers & powertape.
Step 1: Insert, cut & glue
The pipe goes in the tube - this will give extra strength to the shank (Shanky 1.0 broke, that's why).
Cut to the length desired and cut a hole in the pipe to insert the hardware of the handlebar (depends on the type of bar etc. - common sense).
Step 2: Fix & customize!
How attaching the bag? Pass some rope through the shank and the bag, make some loops & finish with a carabiner.
Cool & sturdy, you'll love it!