If it has not been suggested before, can I ask if a slimline lithium battery box is available for the standard UNO?
I keep my UNO in a plastic case and is easy to add a shield on top with a mini breadboard to prototype a complete project on....apart from the battery that dangles off to one side by a wire.
What I would love to see is a clear box for the UNO that has enough room for 2x BL-5C (old nokia) 3.7v lithium batteries under the board, with an on/off switch somewhere and a charging port for the batteries.
Then it would be a portable block unit with no dangling wires, and be self contained for the smaller projects.
I would recommend the BL-5C for the project because they are an outdated format - and therefore cheap! and would allow these batteries to have a useful life. You can always solder to the battery tabs n the box if you wanted to use another format. It would need soldering wires to the UNO board from underneath unless it could be done with sprung contacts onto the power supply socket.
It may be a good idea to blank off that socket with the box design to avoid problems caused by adding external power while batteries are connected to it.
Seems like an ideal project for 3D printing.