The main thing is that you need a good solid plug (mold pattern). Then select a plastic pop bottle of the appropriate size that will just slip over the plug. Cut the bottom off the bottle and slip the plug all the way into it with the propeller opening centered on the bottle neck.

Then shrink the bottle all around by using a heat gun on high temp setting. It's almost impossible to overheat or burn this plastic so don't worry about it getting too hot as long as your plug will stand the heat. Plaster of course or even wood is ok.

Then you cut the excess bottle plastic off flush with the back of the plug so it can slip out but leave the neck on till later. The bottle will have shrunk so tight that you'll probably need to use compressed air to get the plug out. I do this by screwing the cap back on and blowing air into a small hole cut in the cap. The plug will then pop right out. Now you can cut the neck of the bottle from the cowl and trim the motor shaft opening to size.

Paint does not stick well to this kind of plastic so you have to prepare the surface by sanding it with about 220 grit to give the paint something to grip to.

These cowls are nearly indestructable. Ever try to tear or crack a soda bottle? The heat shrinking thickens the plastic so it becomes even tougher. I have made many for different planes this way.

In the picture of the cowl, the one on the left is the ABS vacuum formed cowl that came with the Focke Wulf 190 kit. It's very heavy at 2oz and is not shaped very well. The "cowl" on the right is the plaster plug  which I have done some rough re-shaping to, to try to get it closer to scale profile. This will serve as a mold for a heat-shrunk soda bottle cowl