A little experimentation goes a long way.
鼎anvasses used to be pretty common. They wrap front and bottom to restrict cold air flow.
Try the cardboard first. It is usually not quite perfect, but will usually get the temps up. Running at the correct rpm makes a difference too. Different machines prefer different working rpms to hold/make heat.
Using cardboard/canvasses/winter-fronts is just a matter-of-course with diesels in cold areas of Canada.