That’s a good point. I leave my batteries inside the backpack I use which keeps them sufficiently cool. I haven’t really had to think about it because of this…maybe a panel with a little prop built in to give a battery some shade? I do like the backpack because the bottom is rigid, so it’s always at an angle when I set it down. That’s good up here where the sun is always to the south to some degree - you want to that panel pointing straight at the sun.
I’ve never thought of a white panel. That’s a good idea…