Mark - A wine bottle cork or a rubber stopper of appropriate size and further secured with a suitable type of adhesive tape...?

Or what about a thin walled container inside the bored hole?

Give us some dimensions and the ideas will spring forth... Outside diameter, inside diameter, depth of recess - that sort of thing. Also some idea of the tools/skills readily available to you.

Assume that you will at least seal the exterior of the entire stave with a couple of coats of polyurethane or tung oil or something and that you will at least put some sort of ferrule on the tip to increase wear resistance...

Regards,

Tom