I mostly followed this awesome tutorial.
With minor changes:
(1) I used a single RGB led with a 1 KOhm resistor. The RGB led is interesting because since different colors have different resistance you have a sort of priority of colors. In this case:
If Red is on, then it will show Red independently of Green or Blue status.
If Red is off and Green is on, it will show Green independently of Blue status.
If Red and Green are off and Blue is on it will (duh) show Blue.
This is awesome since I can use blue for the watchdog, green for write and red for error, all with just one led.
(2) I've removed the external crystal by adapting the "minimal setup" shown here.
I still need to wait for my ZIF to arrive and then design/printout the casing + do the soldering, but after many hours I'll be happy because I saved about 1 euro if at all. :)
In the casing, I'll probably add slots so that I can easily plug-in the clock and capacitors in the future.
The capacitor shown of the left of the Nano is required for most cheap ones and must be removed when programming the nano itself, so my casing will also have a "Program Arduino/Program Atmega" switch that disables the capacitor when programming the arduino directly.