reference plane 40 at between about zero degrees and about thirty degrees 0 u00b0 u2266 x u2266 30 u00b0 in some embodiments the right wall plane 48 may be angled with respect to the side wall reference plane 40 at between about one degree and about ten degrees 1 u00b0 u2266 x u2266 10 u00b0 in the illustrated embodiment the right wall angle 52 is about three degrees 3 u00b0 fig5 shows the lip 36 coupled to the front wall 14 the left side wall 22 and the right side wall 24 the lip 36 defines an outer dimension 56 along the x axis a left lip plane 60 running parallel to the left side of the lip 36 and a right lip plane 68 running parallel to the right side of the lip 36 the left lip plane 60 is arranged at a left lip angle 64 with respect to the side wall reference plane 40 the left lip angle 64 may be between about zero degrees and about thirty degrees 0 u00b0 u2266 x u2266 30 u00b0 in some embodiments the left lip angle 64 is between about one degree and about ten degrees 1 u00b0 u2266 x u2266 10 u00b0 in the illustrated embodiment the left lip angle 64 is about zero degrees 0 u00b0 the right lip plane 68 is arranged at a right lip angle 72 with respect to the side wall reference plane 40 the right lip angle 72 may be between about zero degrees and about thirty degrees 0 u00b0 u2266 x u2266 30 u00b0 in some embodiments the right lip angle 72 is.