$fn=50;
l_lower = 20;
h_lower = 2;
l_higher = 10;
h_higher = 1;
h = 5;
b_cable_tie = 5;
color("RoyalBlue", 1.0)
difference () {
  hull(){
    cube([l_lower, l_lower, h_lower], center=true);
    translate([0,0, h - h_higher/2])
    cube([l_higher, l_higher, h_higher], center=true);
  }
  for(rot = [0:1:1]){
    rotate([0, 0, rot*90])
    translate([0, 0, h_lower/2])
    difference() {
      rotate([90, 0, 0])
      cylinder(r=b_cable_tie/2, h=l_lower*2, center=true);
      translate([0, 0, -b_cable_tie/2])
      cube([b_cable_tie, 2*l_lower+0.01, b_cable_tie], center=true);
    }
  }
}