// difference() {
//   cube([150,30,30]);
//   translate([0,5,5]) cube([150,20,30]);
// }


M = [ [ 1, 0, 0, 0   ],
      [ 0, 1, 0, 0   ],  // The "0.7" is the skew value; pushed along the y axis as z changes.
      [ 0.5, 0, 1, 0   ],
      [ 0, 0, 0, 1   ] ] ;
multmatrix(M) {  
  difference() {
    cube([150,30,30]);
    translate([0,5,5]) cube([150,20,30]);
  }
}