I ended up starting a batch of 4 spindles on the lathe. The spindle will accept plates as thin as 3/16ths of an inch, and as thick as 1/2 inch.
There was a spindle which was available on another site, but it did not look robust. All surfaces are true relative to each other within 1/1000 of an inch.
The spacer which rests on the abrasive side is a friction fit on the shaft, to enhance trueness.
Below is a comparison picture between the old and the new style. I wanted a longer shaft on this spindle, so the quill on the drill press didn't have to be cranked down so far.