Do you remember the old vacuum column tape transports of computers gone
by?
They worked by using sensors to keep a loop of tape free of tension
between the two spools.

Also the mechanical solution you have is pretty elegant-look into those
curved fiberglass strips they use to hold reflectors between the
highway that look like venetian blind slats and the fiberglass rods
they sell in home depot to mark driveways for the snowplows.