Door bell power supply (with some additional features)
I bought a 12 volt, 4 amp power supply from eBay, prised open the case and removed the innards. (Long gone are the days when I would make my own power supply with a meaty mains transformer, bridge rectifier and smoothing capacitors or really far back, a nice EZ81 double diode.)
This particular unit has an IEC C14 mains inlet socket making it easy to connect to the mains.
I unsoldered the output lead and connected the supply to a small PCB I made with screw terminals and a couple of 7805 voltage regulators to supply 5 volts to the Pi and perhaps in the future another Pi with a camera. The 12 volts is available for a Triket-controlled LED security light.
The whole shooting match fits into a Hammond case with enough room for a Pi Zero which, in the future, I might include to provide a security camera.
Heat sink - no need for insulation as 7805 tab is ground. Just a dab of silicon grease required.
To make the cut out for the socket, mark out the plastic, cut the verticals with a fine saw then score the bottom cut with a Stanley knife several times then snap off the waste plastic. Clean up with a fine file.
Features like these slots can be removed by paring them away with a sharp chisel. Do it in about three stages rather than trying to cut it all in one go.