These types of sheaths are made from sheets of plastic. My first sheaths were made this way. The sheets of polystyrene were cut into strips and then to length. The pieces were placed on a narrow heating element, folded in half and clamped. Then some finishing work was done. No tooling required.

The ones being made now start out being die cut or stamped out. This can limit you as to how thick of a sheet you can use. The felt lined sheaths I tested were made from a .060″ thick sheet. That’s for both the wide and narrow ones. Ideally the wide ones should be thicker, because they need to carry their tension over a wider area. When I made my sheaths this way the narrow ones were made of a .060″ thick sheet and the wide a .080″.