I bought a 5m cable for the same purpose and it works fine. It had the correct connectors (RJ10) but was reverse polarity* - can't recall which cable was straight through and which reversed, though (original short cable or new long cable). It's the same as a telephone
handset cable .... remember when before cordless phones were invented, and you could get really long cables? In this case, though, I think only two wires are needed.
* That actually works best for me anyway, since I mounted the Accufocus on the reverse side on my ED80.
Edit: This is similar to the one I bought:
http://www.ebay.com.au/itm/6-4m-Coil...item2ec13fe25d