Step 2: Rewire your battery holder
This step is necessary if you use a small enough solar panel to fit on top of the tin. I rescued mine from some cheap solar landscaping lights. They produce more than 3V each in direct sunlight, but that's not enough to charge all four batteries. So I decided to make them charge two sets of two batteries. This is possible with a DPDT (Double Pole - Double Throw) toggle switch. You need to alter the battery holder to make this possible. First, you need to remove the cover of the wire compartment. This was done with a shop knife by cutting off the plastic connectors. Then, just cut the connection between batteries 2 and 3 (Red arrow). Lastly, solder a wire to each connection and run them out the same hole the other wires are coming out. I also removed the switch from the holder because I won't be able to get at it once the holder is attached to the project tin... and you never know when you might need an extra switch anyway.