Jim,

Those are nylon skids just to reduce the points of contact and friction between the oversized sleeve and the wire.
I imagine that sliding them along the wire must be hard, as the manual states there is a tight fit between both.

Just verify that your solution allows completely free sliding movement of the wire within the sleeve or plastic guide, and that the skids cannot slide by themself after a while to the end of the sleeve to interfere with the free movement and full deflection of the elevator and rudder.

Nice model.

Best luck!