Hi Gary
At a guess I would say you are looking for a Losmandy MA (Meade Adapter)
http://www.losmandy.com/access.html about half way down the page.
It is 6" round by 4" high and has the flanges for securing your Digital Drive or Gemini unit.
Secures to a pier by a 1/2" bolt. I have one on the top of my pier and it fits perfectly, will even fit your G-11 when you upgrade, lol
Cheers
JohnG
EDIT: Heres a picture.