I think the main points have been covered:

degrease
spindle not protruding through nut
deeper hole

I think the last deserves further comment. The nut ideally should be sunk
completely in the wood, not just partially. If your mdf isn't thick
enough, glue more layers on the back. The hole should not go all the way
through the wood. The join will be stronger if the nut can bottom out in
the hole. The hole does not have to be a tight fit. Loose is okay, you
will be filling the gaps with epoxy anyway. Expect to true up the disk on
the lathe before attaching the abrasive.

You might want to consider adding some buffing compound to the edge of the
disk and using that surface as well. Some people make the edge a convex
shape and use it to hone the flute. Another option is to have two grits
on the front surface of the disk, an inner ring and an outer ring.

-mike paulson, fort collins, co