This may result in a tight draw, because the mouthpiece has a narrower internal diameter than the plug skelley is referring to. Thought it may work perfectly. If it doesn't you can always figure out a way to chew out the center a little bit and make it wider.
If you use a drip tip, you'll probably want this slice to be on the thinner side so the drip tip fits. Hopefully you have something sharper and cleaner to cut with than the knife below
This is the plug skelley was referring to (one of them, all you need IMO)
The goal is to have something that fits flat and flush on the cup like this:
So take a slice of a carto mouthpiece:
shove it down (the mouthpiece in this pic is not sliced, i'm running low

). use a pencil or something similiar. Just get it flat on the cup, don't invade the wick slots, just kiss the top of them as in the two pics above (2 and 4) (refer to the photo of the black plug flush on the cup)
That should handle your issues.
Note, the wicks in my pick are NOT fluffed, and you don't want your wicks to look like that (that's a fresh one). You want to gently pull them up (gentle, don't use anything sharp) so they cover the wick slot entirely. The coil will be below the wicks, it will look kind of funny. For a better image see the video in that link.