of the detents 46 and 44 for the exemplary embodiment will be described in greater detail below as illustrated two detents may be included on each wall 40 or 42 the use of two detents assists in limiting rotation of the lower channel 18 with respect to the upper channel 14 however one skilled in the art will understand that any number of detents or no detents at all may be used in connection with embodiments of the invention referring now to fig5 7 an embodiment of the assembly of lower channel 18 to upper channel 14 is shown and described during assembly the lower channel 18 is moved from the position shown in fig4 to that shown in fig5 at the start of this operation a formation such as a chamfer 48 see e g fig7 is started over an edge 51 of the upper channel 14 during this operation the expanded region 36 is slid over the lower channel 14 to allow the edge 56 of the lower channel 18 to be pressed against the entrance 54 of the clip 50 this pressing action deflects the clip 50 away from the upper channel 14 to allow the bottom 45 to slide between the clip 50 and the upper channel 14 also during this operation upper detents 46 and lower detents 44 pass over opposing walls 23 and 25 the expanded region 36 is slid over the upper channel 14 until upper channel 14 abuts against the channel region 38 see e g fig7 of the lower channel 14 as can be seen from fig5 the clip 50 frictionally presses the bottom 45 against the upper.