The ATX
power supplies are fairly beefy sources of power at 3.3V, 5V, and 12V,
and some relatively minor amount of 
.
Also, they are cheap and plentiful.
A common approach to this resource rebuilds the entire power supply, removes the cables and attaches the terminals to the casing. Another approach was however chosen, using a separate box as an adapter. This allows using unmodified PS, and rapid borrowing of power supplies. The box can be also used for testing of power supplies, allowing easy attachment of dummy loads and easy powering up of the units.
A 20-pin Molex connector, difficult to obtain as new part, was harvested from an old motherboard, using hot air. Terminals were attached to a box. Spring-loaded clamps were chosen for 5 and 12 volts; while not as secure, attaching wires is quicker. Banana screw terminals were used for all four available voltages.
An ammeter was attached to the ground branch, to indicate the total current (except current between branches, and with the -12V branch current subtracted). A 3-amp meter was chosen on the basis of availability.
A load resistor had to be attached to the 5V branch, as the power supplies tend to not start when this branch is not loaded. A wirewound 47-ohm resistor was chosen; the modifications usually employ 10-ohm 10-watt resistor, but higher value was chosen to dissipate less heat. It appears to work well for at least some power supplies (those tested).
A switch was used for the PWR_ON signal, connecting it to the ground; this signals the power supply to switch on. The 12V line was connected to a 8mm green LED, indicating power-on status. The POWER_GOOD signal was connected to a 3mm green LED, using the PS's own signal for the voltages being within limits. The standby 5V was connected to a 3mm yellow LED.
Color markers matching the ATX-standard wire colors were painted on the Molex connector and the terminals.
Labels
were printed on a laser printer, cut and attached using double-sided tape.
 View from top |  View from edge |  Front view |  Bottom view |
 Inside view |  Inside view, other side |  Inside view, switch and LED detail |  Connector |
 Connector, glued inside |  Connector, from the outside |