Experience has taught me this:
1. Making portable projects with Arduinos requires batteries.
2. Making robots requires an easy accessible OFF switch. (Asimov's 4th law??)
3. It would be great to combine 1 and 2.
4. It would be greater still if it cost nothing to make.
Looking around for something to hold a 9v battery (or maybe 4x AAA cells) I found a box of old mains adapters. The first one I picked up looked about the same size as the Arduino, the second was slightly smaller but had the advantage of a label that read "SINCLAIR ZX80 power supply"
Mains adapter (Wall wart etc.)
2.1mm power plug (If you're lucky that will be attached to the adapter.)
Single pole switch
Wire to connect to externals (e.g. motor controllers)
Small self tapping screws
Step 1: Cut Out and Keep
Take apart the case and save the screws, they may be useful.
First, cut the case to fit the length of the Arduino. (Tip: Measure twice, cut once.)
Then cut to a height that will accommodate the battery. In made this 18mm plus 2mm for the thickness of the plastic. Don't throw away the remainder as this will come in handy.
Remove any obstructions, pillars etc. and sand rough edges.
Step 2: Put It Together
To attach the Arduino to the box I made two L shaped brackets from pieces left from the case. Check the position lines up with the holes on the PCB then glue into place - I used CA but hot glue may hold. When these are solid drill a 1mm pilot hole for the screws.
Drill a small hole for the wires from the power plug.
Note the position of the switch so that it's not under the USB port or the power socket.
If you have a toggle switch drill a hole to size and fit in place. If like me you have a slide switch then make a square hole for the slider and a couple of holes for the mounting screws.
Solder the red wire from the battery clip to switch. Find the wire from the power plug that is connected to the centre pin and solder this to the other terminal on the switch. Solder the black wire to the other wire from the power plug. If you want to power other things from the battery add one wire to switch and the other to the black wire. Cover the connections with some insulating tape.
Step 3: Finito
Clip on the battery and place in the box. Put a piece of insulating tape on top of the battery as it will press up against the PCB. Place the Arduino on top and fit the screws.
Connect the power plug and throw the switch. Sorted.
You can probably see from the photos that I modified the power plug - it stuck out way too much.
The only thing I was going to add was a strip of Velcro to hold it on to my rogue robot - but that would spoil the look of the ZX80 label. I'll have to make another one.