Well, I decided to build a 30" disc sander. I found a faceplate for an old Oliver 66 lathe to use as a hub. It is 12" across, 3/4" thick, takes a 2" x 6tpi shaft, and has 12 holes already drilled in it. I'll use those to screw on a 30" x 5/8" disc.
I have a 1hp R/I motor to drive it with for the time being.