Hey guys, didn't notice this thread when it was fresh so I guess I'll pull a bit of a Captain Hindsight here:
Why not design the board with 20 pin ATX input as well as output so it'll take a readily assembled ATX->AT adapter cable? Saves you cutting the ATX->AT cable and crimping the connector plus the device could then also serve as surge protection for ATX boards. It would also create direct compatibility with premade solutions for other devices that rely on a regular ATX PSU and connector.
Another improvement might be adding compatibility for newer PSUs
in form of an inverter for -5V (that part shouldn't be too expensive I guess) and maybe by implementing +12V -> +5V conversion using the 4pin +12V CPU power connector which will be present on all new PSUs that have a weak +5V rail (more expensive and probably only necessary for later S462 machines). This upgrade may also come in form of a second PCB that clips on the ATX power input of the surge protection board.
The whole assembly could then be mounted into an ATX PSU form factor chassis together with a TFX or maybe even SFX PSU so no extra case space would be needed.
Is any of this still helpful? 😀
Edit: Yeah, that -5V part is already included, I dun goofd. 😉
Also full ATX compatibility would require monitoring +3.3V and +5VSB as well.