I suggest get a dead PSU and replace the inside from a mATX PSU. Looks authentic and will give you nice results.
Or cut the cable off the dead PSU and wire it to the ATX and hard switch it to on/off from the rocker switch. Get someone who is competent with this work to do it for you if you're not; don't mess with electricity, it ends badly.
As a final note I've always found A500 PSU's to be pretty decent and reliable, even though they are older. Guess they were built to last.