A few years ago I looked all over for something like this, without success. Last week I ran across them at etrailer.com and received one of them yesterday. Well made and a perfect fit for the 2.44" OD dust caps on many of our FW trailers with EZ-Lube hubs. They have two other sizes as well.
Part #290-992. Since I painted my dust caps to match the trailer some time ago, I had to find a way to install them without beating them up with a hammer. I made a wood block that does the job, but this will work better and easier.
Even with EZ-Lube hubs, I prefer to take the caps off when pumping in new grease, and removing the old grease that's pumped out through the front bearing.