I can't tell you how Alan did it.  But I had to do this on a lathed flute.  I used a router to cut out an area exactly 1/2" wide and slightly longer than I thought the finger board would need to be.  Then I cut a 1/2" x 1/2"  piece of wood, in this case western red cedar.  I used a circle template to mark a radius on the end of the insert and on the end of the slot.  I used an X-acto knife and a file to cut that in.  I spent a lot of time and care making sure the insert fit very precisely into the slot.  The bottom of the insert should have a concave surface like the inside of the bore.

To install the insert, I pushed a wooden 3/4 inch dowel into the bore (which naturally, was 3/4" DIA), then dropped the insert in with plenty of Tightbond on the edges.   The dowel kept the insert flush with the bore.  After a half hour, I pulled the dowel out so it would not get glued to the inside of the bore.  The glue had gummed up, but had not set.

After the glue set up (24 hours), the insert is standing approx. 1/4" above the surface of the flute.  I used a drawknife, then sandpaper to make it flush with the flute body.

Finally, I went around the seam between the insert and flute body and filled every little gap with Tightbond/sawdust mixture.  This provided the final airtight seal.

It would be more difficult on a limb flute and would require more hand fitting, but I am sure it could be done.

David A