Hi Skeeter, I obtained some steel dual
coils from nhaler.com that were a little larger diameter than usual. They came with a rubber whistle tip that was intended for them. You may be able to take the carto and your drip tip into a hardware store, and rummage
through all the little drawers of O-rings. Perhaps you can find one that is thicker that will take up the space.
You may also be able to cut the end out of a condom cap from a regular carto, and jam the rest of it in there somehow as a sleeve?
Addendum:
Heck, if it came with a condom cap, you could maybe just poke an appropriately sized hole in the end, and use that in a pinch. Pull it up off the end about halfway to minimize possible splatter?