The end plugs fell out of the handle bars on my mountain bike.
I am using my bike for a triathlon this weekend and the race officials want all holes plugged so that the holes are not an additional hazard if there is a crash during the race.
I took an existing SCAD file (http://www.thingiverse.com/thing:13301) and modified it as the basis of this plug. I printed 3/4" diameter plugs, but the SCAD file is parametric so you should be able to make any diameter plug.
The plug consists of two parts; the actual plug and a spreader that is forced into the beveled back of the plug so that the plug is firmly clamped in the tube.
I used a 25 mm M3 bolt and nut as the mechanism to draw the plug and spreader together ... probably overkill but it is a very tight fit!
The nut fits into a slot in the back of the spreader ... I used a soldering iron to 'melt' the nut into the plastic of the spreader.