The rail portions of the frame each have an upright outer wall, an integral downwardly slanted inner wall and {HEAD ~ a hidden outwardly open slot} therein between {TAIL ~ the outer and inner walls} 
The rail portions of the frame each have an upright outer wall , an integral downwardly slanted inner wall and { HEAD ~ a hidden outwardly open slot } therein between { TAIL ~ the outer and inner walls }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL