In all AT PSU I've converted to Amiga use I do the same:
Link blue and white wires directly on the PSU's board and route the remaining two (black and brown) to the switch of the original Amiga brick.
If the AT PSU you are using is the "baby" format, it's dead easy to toss it inside the original A500-sized brick. No need for the fan.
If is the larger AT PSU, then take the Amiga brick's switch and use it to power-on your power supply unit.
Final result here:
