OpenSCAD rounded box

$fa=1;
$fs=0.4;

module box(width, length, height, radius, bevel_radius=0) {
    difference() {
        minkowski() {
            sphere(bevel_radius);
            hull() {
                translate([radius + bevel_radius, radius + bevel_radius, bevel_radius]) cylinder(height, radius, radius);
                translate([width - radius - bevel_radius, radius + bevel_radius, bevel_radius]) cylinder(height, radius, radius);
                translate([width - radius - bevel_radius, length - radius - bevel_radius, bevel_radius]) cylinder(height, radius, radius);
                translate([radius + bevel_radius, length - radius - bevel_radius, bevel_radius]) cylinder(height, radius, radius);
            }
        }
        translate([0, 0, height + bevel_radius]) cube([width, length, bevel_radius]);
    }
}

box(30, 50, 20, 5, 2);