Sadly there is no way to make a mATX 500W dual voltage PSU bought off eBay for $24 to connect to an A500/600/1200 by buying a connector from A4000T.com or using an old connector cord from a dead A500 PSU, and a $2 on-off switch from RadioShack 
dan,
Matx is usually 3 voltages, +5,+12,-12 and is missing the -5(not needed for amiga 500/600/1200) should easily work with 500,600,1200 - at least the ones i built do work
Maybe i misread this?