You asked about a round hole for your boring bars....
As you know the target dimension... make a stepped plug gage with steps in .001+/-.0002 increments x .2 long. The step diameters are easily held over the short distance by stoning them to size. Start .005 under your final size so you know when you're getting close.
Use a split dowel and emery clothe to take the last .001-.0015 out.
Now here's a question for you... why does the boring bar have to go into a round hole? a "V" with a bolt on top clamp is much easier to make.
http://www.kdktools.com/catalog.PDF Refer to page 6, 5-B boring bar holder, for the general idea. Can be made to fit any size.
Ignorance is curable through education.