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
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