Asking for a Friend ..

Since I see so many people wanting to take bike tours with their home office aboard,

and there are some actual electrical engineers on this forum

Anyone got a parts list and schematic to make a external battery pack with a USB standard output

using easily available anywhere, flashlight batteries ?

Why not just buy an AA driven USB powerbank?
goalzero.com/shop/rechargers/guide-10-plus-power-bank/
amazon.com/SODIAL-Portable-External-Emergency-Smartphones/dp/B01GIZM5JO
amazon.com/Verbatim-Power-Bank-Charger-97928/dp/B007MLHS2S
amazon.com/EBL-Functional-Rechargeable-Batteries-Flashlight/dp/B01KZL7YR0?camp=1789&creative=9325&th=1
amazon.com/EBL-Batteries-Function-Batterry-Included/dp/B071XC6MYD
google.com/search?q=4008496810062+OR+57920101441

I am not an electrical engineer, but I think this is the sort of thing your friend is looking for.
https://www.ebay.com/itm/5V-2A-USB-C...-/122090642804

I was trying to use things like that to build a dynohub powered USB charger, but I was blowing out the circuits when I went too fast down hills and overloaded them. But you described just using batteries to supply it so that would not cause the problems I had.

If your friend had enough batteries in series to add up to 12 volts, they could use the USB chargers that plug into cigarette lighter sockets which might be simpler. Then all they need is a battery holder that will hold 8 (or maybe a few more) flashlight batteries in series.

I hope your friend realizes that if they want a lot of power, they could be buying batteries every day if they do not use rechargeable ones. Even if they get rechargeable ones, a dynohub is not going to keep a power user very happy for very long.

https://www.adafruit.com/product/14
I made this years ago. I still use it on super long rides. With the highest quality AA batteries it charges my Garmin 520 several times or more.

You really didn't make clear what you intend to charge with this setup. If you are just talking about charging a phone then yes you can either buy an AA recharger bank ( I suggest one that holds at least four cells ). I bought a cheap two AA cell recharger once ( they are dirt cheap ) and it would only give my phone an extra 30-40% charge. In short, it sucked. I suppose you could DIY using the same circuit but using C cells but the problem is both AA and C Alkiline cells are only 1.5 volts each. If you use C cells to build a USB recharger it is going to make one large recharger. Oh, I almost forgot to mention, some of the cheaper chargers only charge at about a 500ma current rate. Need I say, that really sucks. Most people are going to want something that charges at about 2000ma. , even more important if you are using the device at the same time.

What you are likely wanting is a circuit that utilizes a step up voltage converter ( 1.5volt to 5 volt ) With one of those you could build a battery bank with as many cells as you think you might need ( in parallel ). You just have to make sure that the circuit you buy/use will at least give you something in the 1500-2000ma @ 5volt output range. Just keep in mind that most converters will draw a parasitic current if batteries are left in the charger. If DIY you might want to also include a simple on/off switch where the batteries connect to the charging circuit.