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

  return myBox
}

fnBox = box(3, 6, 10)