Roll Music roll guides.

 

Close-up of one of the guide discs.

I noticed that with some rolls the paper would wander from side to side by a small amount, perhaps up to a millimetre, as it passed over the tracker-bar.  Although this seemed to make no noticeable difference to the sound volume I felt guides would be a worthwhile addition.  They are turned from 3/8" round brass drilled to be a tight fit on the rod with a saw-cut to allow easy adjustment.   If you do not have a lathe, other methods could be used instead.  Wooden or metal guides, suitably shaped and attached to the side of the tracker-bar would serve just as well.