of the inner surface of the outer perimeter wall 18 to help align and properly secure the cap 86 onto the base 14 cap member 86 also includes a resilient relatively flexible depending latch arm 108 the lower end of which has a forwardly projecting prong 110 which engages a rearwardly projecting prong 112 on the outer side of wall 20 to temporarily hold the cap 86 in place until a fastener 66 is inserted into the opening 64 to firmly lock prong 110 and prong 112 together cap member 86 and base 14 can be formed of a variety of materials but are preferably each made from a strong durable thermoplastic material such as abs or polycarbonate cap 86 and base 14 are each preferably made by injection molding the clear transparent portion or portions of lens 48 are preferably made of a strong durable thermoplastic material having excellent optical clarity such as polycarbonate or polymethylmethacrylate any remaining portion of the lens member which need not be transparent can be made of any suitable thermoplastic material if the two halves 48a and 48b of the lens member 48 are made of different materials it is preferred that the materials be compatible so that they can be fused together such.