Here are a couple of pictures of the plugs that I made. Foam covered with balsa, shaped and then varnished, etc. I don't remember exactly which bottles I used, but seem to recall that I used a one liter pop bottle for the Dazzler and a wide mouth apple juice bottle for a larger cowl - just large enough for the plugs to fit into the cutoff neck end. I used my monokote heat gun to shrink the pop bottle. One thing that I did that you may wish to consider is - don't throw away the bottle caps. Instead, put a tire valve stem in it and then after shrinking the plastic put the top on the bottle and pump air - I have a compressor - in the valve stem and this will expand the plastic away from the plug. See third photo.