an enclosure having a plurality of holding members arranged side by side on an inner surface thereof, a support member being provided at one end of each of the holding members, and a locating member being connected to each of the support members; and each of the locating members being provided at predetermined positions with a through hole each;

a plurality of connecting units each being formed at a first end into a sleeve portion for locating on the holding member, the sleeve portions each being formed on at least an end surface and a top surface with an opening and an engaging hole, respectively; a second end of each of the connecting units opposite to the first end being formed into an angled portion for locating between the support member and the locating member; and a plurality of clamping elements being arranged on each of the angled portions corresponding to the through holes on the locating member;

a plurality of holding-down members, each of which including a curved pressing portion downward extended through the engaging hole formed on the top of the sleeve portion of the connecting unit to abut against an inner surface of the sleeve portion, and a wing portion outward extended from a middle section of each of two straight edges of the curved pressing portion; and the wing portions being rested on an outer surface of the sleeve portion along two opposite edges of the engaging hole; and

a plurality of connectors provided on one side of the enclosure to separately connect to the clamping elements arranged on the second end of the connecting units.