I've often thought about this, Jo. At first, the answer seems to be just a flat piece of plastic to go over the cab, resting on the top frame of the windows at either side, the ledge at the rear and the top of the windscreen and held in place by a right angled piece of plastic on either side projecting down the inside of the top of the window frame. However, when you have the crew in place you realise that their helmets project slightly above the top of the window frame so the roof piece needs raising by "spacers" at the edges, but not so high as to obscure the blue warning lights.
Hope this explanation is clear, I'll try and do some pics tomorrow.