How crafty do you want to get, and do you care about clearance on the tops of the bars for your fingers?
Ultimate cheap is scavenge a couple of blinky light brackets for the bars, and disassemble the light housing to use part of that screwed to your bag for the other half. Viola built in quick release function.
Otherwise I’d go for one of the many Amazon/Ebay/Aliexpress “accessory holder” bracket and tube assemblies. Downsize the tube to something that works with Ortlieb hooks that you mount on the bag side.
