difference(){
cube([sonar_holder_length, sonar_holder_width, sonar_holder_depth]);
translate([sonar_holder_length / 2, sonar_holder_width / 2, -0.05]){
difference(){
cube([sonar_holder_length, sonar_holder_width, sonar_holder_depth]);
translate([sonar_holder_length / 2, sonar_holder_width / 2, -0.05]){