If weight is of the essence, you can make one from plastic.
Make a foam plug like Andy said by carving/sanding a chunk or sheets of foam. I like to glue it to the firewall with hot glue and just sand with ~60 grit sandpaper OUTSIDE(foam dust is terrible).
Then cover the nose of the plane with masking tape where you want your cowl to cover. This part is similar to fiberglassing. Some also apply car wax to make it come off easier. Now, instead of putting glass cloth and resin, use paper mache. i.e. paper dipped in a glue/water mix. Try to use as little glue as possible so it dries quicker. Once all dry, pull off the plane. You might have to dig out the foam. It will be kind of flexible. Then fill with plaster of paris, and once cured, you can tear off the paper mache. Voila, a solid plug for shrinking.
For the shrinking, there's lots of info about that around. To shrink with a bottle, you need to find a PET plastic bottle that fits around your plug. Then you can heat with a heat gun to shrink. Or I've heated it over an electric stove.
Actually, if you have a vacuum former you can get away with just using the foam plug. If you're shrinking a bottle, the foam will get crushed by the force of the shrinking bottle and will deform.
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