Referring to FIG 4, {TAIL ~ triangle shaped fusing guides} 60 are formed on the surface of {HEAD ~ the inner cover} 40 to be easily fused with the outer cover 70 
Referring to FIG 4 , { TAIL ~ triangle shaped fusing guides } 60 are formed on the surface of { HEAD ~ the inner cover } 40 to be easily fused with the outer cover 70
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH