cornerfillet.scad by church

filletDiam = [10,10,10]; // [0.1:0.1:30]
debugOn = false;

/* [Hidden] */
$fs=0.5; $fa=1.0;

module cornerfillet(cfx = 15, cfy = 25, cfz = 10) {
  smidge = .001;
  cfr = max(cfx, cfy, cfz);
  rnd = cfr - sqrt(2) / 2 * cfr + smidge;
  resize([cfx, cfy, cfz])
    difference() {
      cube(cfr);
      hull() {
        if(debugOn)echo(cfx, cfy, cfz);
        for (r = [[0, 0, 0], [-90, 180, 180], [180, -90, 180]])
          rotate(r)
            translate([cfr + smidge, cfr + smidge, 0])
              rotate([0, 0, 180])
                linear_extrude(height=smidge, center=true)
                  intersection() {
                    square(cfr + smidge);
                    circle(cfr + smidge);
                    }
        translate([rnd, rnd, rnd])
          sphere(r=rnd);
        translate([cfr + smidge, cfr + smidge, cfr + smidge])
          sphere(smidge * 2, $fn=1);
      } // end hull
    }
  }
if(debugOn) cornerfillet(filletDiam.x, filletDiam.y, filletDiam.z);