Hi Hans,
This has been like a bible to me to get started on the Mega > Nano Master/Slave.
Can I ask, you have the 12v power, is that to power your magnetic switches, the only thing I want to power are about 3 LEDs so I shouldnt need the 12V line??
How do you power the Nano's, does the 5v out from the VIN pin on the Mega work?
Also, do you have a picture of your junction box? I assume it will be as tidy as the rest of the nice work!