Pour liquid silicone in each roller and wait till it hardens. Use
this guide as a help, despite it is for bigger wheels. Peel the outside surface and expose silicone. String 15 rollers on a wire of appropriate length and diameter of 2 mm. If rollers do not rotate frictionless, use 2 mm drill to widen axle holes. However, do not increase them too much. Otherwise, the rollers will scrape wheel's body and the motion will be impaired. Sand the open side of the roller so that it becomes flat. Put the whole string in the bottom half.