A molded plastic housing is provided with elastic support members extending in a hollow space, and elastically supporting members. These members have an elongated edge running from a first to a second lateral face of the housing, and accessible through a window. A pre-clamp is assembled to the housing, by placing leg portions on either side of the hollow space, with a base portion extending across the edge of the member. The leg portions are deformed to cooperate with the bottom face of the housing to hold the member in the housing.