This case uses 5 18650 batteries connected in parallel. The case can be made waterproof by inserting a 1.75mm nylon based flexible filament around the inner lip of the base. 

Use a solar panel that is 110x60mm and is rated at exactly 5V. 

Required parts: 

TP4056 charging module

110x60 5V solar panel 

M3x40 screws (6) 

M3 nuts (6) 

Rocker Style Switch (2) 

MT3608 DC-DC Converter