Another cool find. If you decided to go to AC rounter and still want to have automatic on/off for router, I found this socket trigger relay on amazon for $28. Just need to connect the spindle signal and ground.
For trigger on, the spindle speed should be greater than 10k, e.g. gcode: M3 S10000, else it wont trigger PWM high value for the relay.
No need to build one. 
