..."all Bokuto are not created equal"...
For every ryu-ha in Japanese Swordsmanship, there is a unique bokken involved. These different bokken geometries have a different height and width to their specific bokken. Therefore; it is important to measure your bokken at the point where the tsuba will rest to insure you have a proper, tight fit.
Measuring your Bokken
There are two measurements that are critical to the fit of your Leather Tsuba to your bokken and should be measured at the spot where your tsuba will rest. Using a caliper is best for taking these measurements.
A: The measurement of the height, which is the distance between the mune and the ha of your bokken.
B: The measurement of the width, which is the distance between the right and left shinogi.
You want to have a "press fit" that is tight. You may have to force the Leather Tsuba into place. A generous coat of Teak oil or Mineral oil on the bokken will help. Make sure you wipe the excess oil off with a clean rag when finished.
Some folks like to open the center hole a little more by using a Dremel grinder with a sanding attachment. If you do this, be careful to check your fit as you go along so you don't open the center hole too much.