shown as having a generally plane rectangular configuration the short ends 66 and 68 of the cap member 64 are each provided with a normally extending latch means 70 and 72 respectively arranged to releasable engage a respective lower slot 50 of the base member arms 46 the arms 46 are designed so that as the cover member 56 is seated firmly over the base member 26 the arms 46 will extend through the apertures 52 in the cover member 56 sufficiently to expose a sufficient portion of the lower slots 50 above the top surface of the cover member 56 to enable each of the latch means 70 and 72 to enter and engage a respective one of the slots 50 the engagement between the cap member latch means 70 and 72 and the slots 50 in the arms 46 is accomplished by applying a downward pressure against the cap member 64 causing the latch means 70 and 72 to contact and deflect the arms 46 outwardly sufficiently to permit further movement of each of the latch means 70 and 72 against the inwardly facing surface of a respective arm until each latch means is aligned with and engages a respective slot 50 substantially as shown in fig5 and 6 the.