I didn't know what to order either, so I made one from 2" x 2" x 1/4" thick angle iron. I made sure the face of the platen was flat (granite surface plate), after which I welded an edge to the bottom of it (sticks out about 3/16" but you can just drill a couple of holes and tap two screws in so that they sit parrallel to the edge of the squared-off angle). This is necessary to support the glass platen that will be lued to it later. Then I matched the radius on my large wheel (10") and gave it about a 1/2" gap and cut/gound the other leg of the angle iron so that it would sit above the wheel, without interfering with it's rotation. Then simply made two slots for two cap screws with locking washers on them to bolt into the multi-platen. When all was done, I JB-Welded a pyroceram platen on it, let it dry for 24 hours, and bolted it to the multi-platen.
It might seem like a lot of work, but it only took me about 2 hours, and it's pretty easy. If you have the flat platen arm for the GIB, this is even easier, because you don't have to worry about the radius of the wheel - it just forts in there squarely. Shoot me an email if you have more questions.