Works for mountain or road bars, woohoo!
If you've lost a bar end plug, replace that puppy ASAP! And if you're going to replace it anyways why not get a fancy US made, anodized one? Or just go there right from the get-go! Compression style plugs stand a better chance of staying in your bar ends as you can snug them down tighter rather that just relying on the grippiness of the rubber or plastic of the plug itself.
Protips: For drop bars, do not tuck the beginning of the bar time into the bar end. Leave that space free and clear. If you're still worried about the bar plug staying in or the torerances don't seem quite snug enough on your particular bar, you can wrap a little strip of fabric (i.e. handlebar tape, a bit of your roommates t-shirt) and secure it with a dab of super glue gel, one wrap around the compression part of the plug, should do the trick. Just to bulk up the outer diameter of the plug. Fiber grip might also be necessary to get the outer part of the plug to grip the inside of the bar enough to get a secure fit or to keep the plug from rotating as you tighten.