wheel_thick=6;
diameter=21;
thick=3;
drill=3.2;
height=16;
w = wheel_thick+2;
l=13;
l2=l+8;

//-- wheel
*translate([0,0,-drill/2-1])
rotate([90,0,90])
difference() {
  cylinder(r=diameter/2, h=wheel_thick,$fn=50,center=true);
  cylinder(r=drill/2, h=20, $fn=50, center=true);
}

rotate([0,180,0])
difference() {
  union() {
    //-- One side
    translate([-w/2-thick/2,0,height/2])
    cube([thick,l,height],center=true);

    //-- The other side
    translate([w/2+thick/2,0,height/2])
    cube([thick,l,height],center=true);

    //-- Top part
    translate([0,l2/2-l/2,thick/2 + height])
    cube([w+2*thick,l2,thick],center=true);
  }

   translate([0,0,drill/2+1])
   rotate([0,90,0])
    cylinder(r=drill/2,h=20,center=true,$fn=50);

    translate([0,-l/2+l2-drill/2-4,0])
    cylinder(r=drill/2,h=2*height+4*thick,center=true,$fn=50);

}