If you google AA solar battery charger you will find lots of videos and reviews. It's milky sunshine this morning but the charger test light shows full charge. I angle it with a small book so that it's directly facing the rising sun and then if I'm around during the day I shift it slightly so that it's more facing south west, but that's just me being obsessive - not really necessary.
I put it on most days when the car isn't being used. Very easy if your EOBD port is fairly accessible. There is a connector for the ciggy lighter socket, but that's dead when the ignition is off so not much use. By the way, I'm not sure that the jump start terminal in the fuse/relay board can be used for charging - I think it's just for jumping, but you'd need to check.