And I might put together something like that in the near future. I was taking apart cheap power banks for the board, but then I found pic related on Amazon. $10 for 10 boards. You can hook up regular alkaline batteries like AA’s or D cells, or put some 18650s in there. Get a cheap little battery holder, solder 2 wires, and you’re good to charge up whatever devices that use a regular 5V USB charger with whatever batteries you can drop in your pack.
Only issue is I don’t believe these boards have protection. With AA’s, you could run em til they’re dead and it’s fine. But with Li-Ion like 18650s, you would have to watch the voltage. They sell tiny voltage readouts for a few bucks a dozen and could hook that up between the battery and the board and unplug when the power source battery gets low or you could spend a couple $$ more anf get the same boards with protection built in.