Basic duct transition using hull in OpenSCAD

duct.scad

fan=120;
ductl=110;
ductw=40;
h=150;
wallTh = 2;

module transition(h,l1,w1,l2,w2,eps=0.01) {
  hull() {
    cube([l1,w1,eps], center=true);
    translate([0,0,h-eps]) cube([w2,l2,eps], center=true);
  }
}

difference() {
  transition(h,fan+wallTh*2,fan+wallTh*2,ductl+wallTh*2,ductw+wallTh*2);
  translate([0,0,-0.01]) transition(h+0.02,fan,fan,ductl,ductw);
}