$fn=200;
L=20;
W=15;
H=5;
R=60;
difference() {
  translate([-0.5, 0, -1]) cube(size=[L+0.5, W, H+1]);
  // circle of radius R through 2 points (0, H) and (L, 0)
  translate([L/2 + sqrt(R*R-L*L/4-H*H/4)*sin(atan(H/L)), -0.1, H/2 + sqrt(R*R-L*L/4-H*H/4)*cos(atan(H/L))]) scale([R, 1, R]) rotate([-90, 0, 0]) cylinder(h=W+0.2, r=1);
}