HiLetgo 3pcs TP4056 Type-c USB 5V 1A 18650 Lithium Battery Charger Module Charging Board with Dual Protection Functions

Why use a battery bank? You already have the 18650 batteries, just sort of "make one" with the addition of a charging/output module. That's all those battery banks really are. If you're using a solar panel with a USB out, you can use this: https://www.amazon.com/HiLetgo-Lithium-Charging-Protection-Functions/dp/B07PKND8KG If you're using solar panels where you need to wire it up, get a board that has the proper voltage specs for your panel. As an example, if it's a 5V panel, you can use this: https://www.amazon.com/WatangTech-Solar-Manager-Compatible-Raspberry/dp/B0FQP915R9