Mike, I would take a 1/2in dowel rod of your choice and begin at one end, (to become the lower end) and using what tooling you have, ( I start with a belt sander and then go to a rasp,) reduce the dowel to the 3/8s size you need. Moving up the rod as you go, slowing as you approach the upper thimble. Finish with sand paper using a shoe shine motion. It's very simple to do and can be done in about a half an hour, maybe 45 mins. I usually add a tip on the lower end to use when cleaning in the field after checking the RR hole for clearance!