Abstract: The frame 10 consists of one single part and is provided with a rectangular aperture each at its front and rear side the aperture being smaller in size than the covers. Two opposite edges 26, 30 of at least one aperture has a distance which is equal to or slightly larger than the width of the covers 12, 14. Parallel to the two other opposite edges 24, 28 of the aperture, a groove 34, 36 of U-form cross-section extends in the interior of the frame one of the two grooves, for instance 36, being deeper than the other 34. In the proximity of the deeper groove 36 a fixing means 42 is provided.