Actually I don't know. A simple rubber cup should be too much easy to lose when you remove it to use the earphone... I'm wondering if it's not better to remove lint whenever it goes inside...
I looked into the idea of a dust plug, but found them to be expensive and -imho- gaudy (like this). The only time I had this happen to me I vaguely remember, just prior, feeling like I had to exert more pressure than usual to insert my headphone jack. Hindsight I realize there was something in the way and I should have immediately stopped. I also am more careful about not putting it into a pocket that might have something like a kleenex or other lint producing material in it.
Since I've only had it once in about 8 months, and have a pretty good understanding of how I contributed to that situation, I've decided to not bother with a plug that I then have to manage whenever I want to use the headphones.