I built a custom power connector for the Micronik A1200 busboard from two old AT power connectors (you know, the six pin kind). It appears that they can be cut down to the desired length with a sharp knife, and they fit snugly in the connector both ways. You'll have a two-part connector but who cares..
Also, I have some good news for anyone building this connector. Plugging it the wrong way does not seem to damage your setup (it just won't boot). Yeah, I tried it :-D