Thanks Bob,
I ordered one 6" DIA, BY 1/2" THICK aluminum disc from mcmaster, should have ordered 2. Oh well, live and learn.
Thank you for the info.
RobS
Go pick up some baltic birch plywood from a local lumberyard or home center and just make the silly disc yourself ... better yet, pick up some large diameter nuts as well that match your lathe's spindle shaft and use multiple layers of the plywood to give enough depth to embed the nut inside the plywood so that it has good holding power on the wood.
Nuts of that size are under 2 dollars each (the 3/4" x 16 one for my lathe costs me around a dollar each), and baltic birch ply has so many uses around a shop ... it's very strong, stable, and has very very few voids between the layers.
Another option would be a piece of leftover Corian from a countertop project, in several layers. Glue the layers together with plumbing PVC pipe glue and clamp it up well (book press or just stack heavy stuff on top).