I have laid out a simple 3" by 3" PCB with the following features:

- takes 20- or 24-pin ATX power supply connector as input
- routes the GND, +5V, +12V, and -12V rails to appropriate connectors for attaching the original Xerox 6060 / Olivetti M24 / AT&T 6300 internal cables
- boosts the 12V rail to 15V using a XL6009 IC (this can be disabled through a jumper if not needed)
- has a 3-pin fan connector to power a 12VDC fan
- has a jumper cluster to select four possible speeds for the fan
- has a jumper for the ATX "power on" switch (if jumpered closed, the ATX power supply will turn on as soon as the mains switch is operated)
- has three status LEDs (5V stand-by, main 5V, and boosted 15V)
- has a 3.3V post for future expansion / in case you need this voltage for something else (low amperage only)

In my mind it is meant to be attached to the side of the original power supply enclosure, with the ATX cable attaching from the back and the Xerox cables attaching at the front. This way it will also receive some airflow from the fan.