The holder has at least two holding brackets (4) each with a guide arm (6) parallel to a base (2), and a vertical holding arm (8). The guide arms are attached to the base by a tightening device including a screw (12) and a washer (14). The guide arms can be moved relative to each other and locked by tightening the screw. Each guide arm may have a longitudinal slot (10), through which the screw passes. The holder may have a guide part with grooves in it to guide the guide arm.
