I've made them before from a small test-tube stopper. I've also [slightly] enlarged the hole and tapped it, and made a short plug from a section of a bolt of the same thread, and a slot across its face for a flat-blade screwdriver, to turn it into place and set it with Loctite. I first did that in 2012: the bike just came back last year for some more specialized work on it, and the little plug is still there, in the same position I set it at 10 years ago! Here's a picture of that one.
