Wow, the chinese are actually making them alreadyvs with a large battery and assuming it's flexible (non-proprietary) a custom solution (not hard to do at all):
52VDC --> DC-DC Buck converter (5VDC, efficiency losses, but less than a buck-boost) --> USB power.

5.96US $ |10V 80V Waterproof Electric Bicycle Motorcycle Handlebar Dual USB Charger Power Adapter With ON OFF Switch for Mobile Phone GPS|Car Chargers| - AliExpress
Smarter Shopping, Better Living! Aliexpress.com