found by using a caliper on a 45 RPM record adapter

Follow-up note:
After submitting the above figures, I have also measured the spindle on the turntable, the hold in a 33-1/3 rpm record, and the hole in a 45 rpm record.
All of the measurements (by slide caliper) are consistent with the above data, so obviously the tolerances are very small.
You'll need to make the inner hole a little bigger to make sure it goes over the spindle, and the outside diameter slightly smaller to make sure the record will go over it. But the differences have to be really slight (less than .01", I would suggest).
Good luck!