I cycle a lot and I placed quite a few electronic accessories on my bicycle handle bar. My issue is that I use a bicycle pouch to hold my powerbank to power my devices and is quite troublesome to remove and mount back the pouch on my bike each time I leave my bike at the train station. So instead of using a pouch I decided to create a simple holder for my powerbank which can easily place my powerbank.
What you need:
1. Powerbank (you can easily get this 2600mAh powerbank model in eBay for about $10, see picture of powerbank)
2. 3D printer to print the holder
Attached is the STL file for you to 3D print. If you don't have a 3D printer you can engage someone with a 3D printer to print it for you. It will take about 1.5 hours to print one and cost less than $2. The diameter frame is currently set at 29.6mm, you can adjust accordingly to your diameter frame using a 3D modelling software (I use 3D blender software).
$12 = $10 (powerbank) + $2 (3D print holder)
Some eBay listings of this powerbank does not come with the battery and it usually priced less than $2. So just make sure it comes with a battery you purchase it.