Spherometers go for $200USD or so, but the dial indicators only cost $20.  I made this to help me get closer to accurate while grinding a telescope mirror.

Radius is 25mm.

Additional components required:
1 - Mituyoyo Dial Indicator  (8mm neck)
1 - (optional) M3 Grub Screw
3 - Allen head #10-24 bolts - ½ inch is probably good enough
3 - Steel Ball Bearings.  I'm using 3/8".  They will work, but I could use some half the size or smaller if I had any laying around.
6 - (optional) #10-24 nuts

superglue or equivalent 

This print was made with NO TOLERANCE.  It is meant to be tight, as mirror makers are trying to get accuracy down to molecule/wavelength measurements.  You will probably need to core out the center (Dial Indicator) hole to get your gauge to fit, and scrape out the recesses for the three nuts.  The extra three nuts are to lock the position of the Allen Head bolts to the fixture.  Or you could use LocTight.
 Use a drop of stuporglue to hold the ball bearings in the heads of the Allen head bolts.