$fn =100; 

difference(){
union(){
difference(){
difference(){

// outer shell    
union(){
   translate([10/2,0,0]) cube([10,75,15],center=true);  
    
difference(){
cylinder(d=75, h=15, center = true);
    translate([75/4,0,0]) cube([75/2,75,16],center=true); 
}
}

// inner schell
translate([0,0,-1]){
union(){
   translate([(10-3)/2,0,0]) cube([10-3/4,75-3,15],center=true);  
    
difference(){
cylinder(d=75-3, h=15, center = true);
    translate([(75-3)/4,0,0]) cube([(75-3)/2,(75-1.5),16],center=true); 
}
}
}
}
translate([10,0,(15/2)]) rotate([0,90,90]) cylinder(d=10, h=10, center = true); 
}
difference(){
     
translate([-1,0,(15/2)-(9/2)])rotate([0,15,0])cylinder(h=9,d=6+3, center= true);
    translate([-20/2,-20/2,15/2])cube(20);
}  
}
translate([-1,0,(15/2)-(9/2)])rotate([0,15,0])cylinder(h=9+4,d=6, center = true); 

translate([-23/2 -18 ,-22/2,5])cube([23,23,10]);
}