Off the top of my head, there are a couple approaches to doing what you want.
Find a 9x12 with a Rapid Rectilinear, Anastigmat, or other symmetrical-type lens design and remove the front (or rear) lens group. This will double your focal length. The camera will be about a stop slower than the aperture setting when you do this. Stop it down to minimize the aberrations induced by breaking up the symmetry.
Acquire a 4x5 camera with lens board and your favorite portraiture lens. Acquire a 4x5 plate holder, and create a 9x12 adapter out of your favorite constructuon material (I would use a thin sheet of plastic shim stock and cardboard).
Whatever you do, I make new 9x12 plates if you want to shoot them. See link in my sig.
-Jason