Thread the Uni knot leader in to the plastic worm the way you always have, starting 1/4 to 1/3 from the bottom/tail of the plastic worm. After threading the Uni knot leader in to the plastic worm, the Uni knot on the leader should be about 1/4 of the way down from the top/head of the plastic worm.
When you do this, doesn't the stubbly uni-knot score the leader tract within the worm?
I was thinking a one-way "barb" would be less damaging to the inner tract. It would tend to slide thru with minimal resistance when sliding the worm along in one direction, but "bite" into the tract to resist movement when trying to slide the worm in the opposite direction.
Scoring the leader tract? Not really, the tag ends just fold over while threading and the tag ends will pop out to 90 degrees inside the plastic worm by just slightly reversing the threading the opposite direction.
By changing the lb. mono. line used for the Uni knot, larger or smaller Uni knot, you can change how much Uni knot friction you want on the leader. Use the smallest lb. mono. for the Uni knot to achieve just enough friction to keep the plastic worm from scrunching and scoring of the threaded hole to a minimum. The heavier lb. test leader (more friction), the lighter mono. Uni knot line piece needed, the lighter lb. leader (less friction), the heavier mono. Uni knot line piece needed.
These mono. lb. leader formulas work most of the time:
10 lb. leader - 12 lb. mono. for the Uni knot
12 lb. leader - 10 lb. mono. for the Uni knot
15 lb. leader - 8 lb. mono. for the Uni knot