You can also use plastic soda bottles for cowlings. Since they are "blow molded" (heated with air forced into them to expand them to a given shape), They can be re-heated to shrink them down onto a plug. Taking the shape of the plug. Simply cut the ends that will shrink smaller than the plug and presto, a new very lightweight cowl.

I have used a balsa block to carve the shape since it can be done very quickly. Sand it smooth, cut off the bottom of the soda bottle (of appropriate size to fit the plug easily). Apply hot air from your heat gun (covering gun works fine). The bottles will only shrink down to about twice the diameter of the mouth. Once the bottle is removed from the plug, you can cut off the mouth which provides a nice pass through point for the prop shaft.