Just a thought; you might check with FLAPS to see if they have a small tailpipe expander. Normally used for enlarging a tail pipe (so it will fit over another pipe of the same size), but I don't see any reason a small one couldn't expand a dust cap.
If you have a drill press or a lathe and a piece of hard wood, you might be able to make an expander that would work. Turn it to just fit into the cap, then cut in half and drive a tapered punch through it to expand it inside the cap.