$fn=20; //How detailed should the sphere/cyl-s be.

holder_height = 2.5;

module rounded_cube(size = [1, 1, 1], center = false, radius = 0.04) { //Radii should be lesser than half of the min(size[0],size[1],size[2])!
    // If single value, convert to [x, y, z] vector
    size = (size[0] == undef) ? [size, size, size] : size;

    translate = (center == false) ?
        [radius, radius, radius] :
        [
            radius - (size[0] / 2),
            radius - (size[1] / 2),
            radius - (size[2] / 2)
    ];

    translate(v = translate)
    minkowski() {
        cube(size = [
            size[0] - (radius * 2),
            size[1] - (radius * 2),
            size[2] - (radius * 2)
        ]);
        sphere(r = radius);
    }
}

color("red"){
    union(){
        difference(){
           rounded_cube([3,3,1]);
               translate([1,1,0.5]){
                   rounded_cube([1,1,2]);
              }
          }
        translate([0,0,1]){
            rounded_cube([3,1,5]);
        }
        translate([0,0,6]){
            difference(){
            rounded_cube([3,3,1]);
                translate([1,1,-0.5]){
                    rounded_cube([1,1,2]);
                }
            }
        }
        translate([0,0,2]){
            difference(){
            rounded_cube([3,3,1]);
                translate([1,1,-0.5]){
                    rounded_cube([1,1,2]);
                }
            }
        }
        
        translate([0,0,1]){
            rounded_cube([3,1,5]);
        }
        translate([1,1,-holder_height+0.1]){
            rounded_cube([1,1,holder_height]);
        }
    }
}