again in close proximity to the lower portion of the cover 44 is a second supporting strap 52 which is welded or otherwise secured to the outer surface of the wall 20 and to the inside of the cover plate 44 the respective supporting straps 50 and 52 are spaced from each other an amount sufficient to just permit the router bit 26 to be slidably received therebetween with the front and rear edges of said bit sliding between contact surfaces of said straps 50 52 a plurality of openings 54 are formed through the walls 20 22 in close proximity to the leading edge 24 with each of said openings having a countersunk portion 56 fig3 the openings 54 are formed on a common axis in a row and are spaced apart an equal amount which spacing coincides with the spacing between adjacent apertures 40 in each row of apertures in the bit 26 a router bit 26 is threaded between the straps 50 52 and between the cover plate 44 and the wall 20 until the bottom row of apertures 40 in the bit 26 align with the openings 54 in the wall 20 a plurality of plow bolts 58 in this case four are passed from the inside of the wall 20 through the countersunk portion 56 and openings 54 and through the aligned apertures 40 in the row of apertures formed along the lower end portion of the router bit 26 nuts 60 are threaded onto the ends of the bolts 58 and are tightened thereon so as to positively secure the router bit 26 to the lower protion of the side 20 with.