Epoxy, and make sure it is not a thick one, I use west system 205, heated for a few seconds in a microwave to about 90 degree F, not higher as that will kick the reaction too fast and it will harden WAY too soon. If it's too thick and you push that plug in the hydraulic pressure will split the shaft right along those splines. Make sure to put glue relief grooves going around it(rings) and then 2 of them lengthwise so the excess epoxy can escape without building up pressure. I use a Q-tip to wipe the epxoy around the inside of the bore and let it sink in a couple minutes, then add a bit more, and also coat the plug. A bit messy, so put down a cover for your lathe bed. ****IMPORTANT***** Insert plug slowly, let the excess escape as you push it in....too fast and you will split the shaft!!!!
I also dry fit the plug, and when seated fully, I use a pencil to mark the plug where it meets the shaft so when installing with epoxy, I know when it's seated fully, as the pressure can make it feel seated but actually isn't. Hope this helps,
Dave