Try a piece turned to slip in with 10 thou clearance, then drill & tap it then cut it at about a 60 degree angle. then drill the remainder oversize and tighten with an allen bolt and pull it out.
The part you cut off has to be about 1/2 or 3/8 on the shorter side....it will look like what holds an old bicycle handlebar stem in.
If it is a blind hole, turn a piece of bar stock to about 3 thou clearance, put some grease in the hole, slide the bar in till it bottoms on the grease, now smack it with a hammer..