Ever have a handlebar end-cap that just didn't want to stay in?
I've lost quite a few over the years. How about you?
I've seen most "fixes" consist of electrical tape wrapped around the end-cap and shoved back into the handlebar. This works great, but eventually that tape will start to slide off. It will get hot and gooey and fall out. You'll probably notice it a few times and get your fingers sticky trying to put it back in, but the eventually the end-cap will turn up missing.
Using a rubber band has worked well for us! Just wrap a normal sized rubber band around the end-cap like you would a 7 year-olds pony tail. Once wrapped, shove back into the handlebar.