Pre-made rod grips are available, but I chose to buy cork rings. This fiberglass blank is .435 to .403 diameter in the span of the grip, and I thought it would be difficult to enlarge a 1/4 inch bore (6.5 inches long) with just a rat-tail file.
An HSS boring tool did a reasonably clean job of enlarging the initial holes in the cork rings. And, I could decrease the bore a little on each successive ring to follow the blank diameter.
It is a rather unusual tool that I show here. I am not certain that it was designed for boring, but it worked.
Then came a nasty surprise. After I glued on the threaded part of the reel seat (5 minute epoxy, of course), I saw that the last thread was not fully formed, and that I could not screw on the locking ring.