fn box(h, l, w) {
 myBox = startSketchOn(XY)
    |> startProfile(at = [0,0])
    |> line(end = [0, l])
    |> line(end = [w, 0])
    |> line(end = [0, -l])
    |> close()
    |> extrude(length = h)

  return myBox
}

fnBox = box(h = 3, l = 6, w = 10)