Magne

Using the plug as a male mold for the balsa works very well.
I have done this a couple of times, using only water to soften the wood. No need for ammonia.
The first time I tried, I soaked the wood thoroughly with the shower, and tied it on the plug. Because of ecxessive use of water, the wood stretched quite a lot, and cracked a couple of places when it dried out, as it became too tight on the plug.
This was however easily glued together with superglue after taking the balsa off the plug when dry.
The second time, I attached the wood to the plug first, then sprayed water onto it. This time the wood curled up a bit on the "flat" parts of the plug as it swelled.
Next time I would therefore apply water first, but use less than the first time, and thereafter attach it to the plug.
When dry, the "shells" will fit very neatly into the mold. I use a marker pen to draw a line along the mold flange, and cut/trim the wood a few millimeters (1/8 - 1/4" inside the line. Thereafter I chamfer the edges all around, to allow the inner fiberglass skin to attach to the outer skin along the edge.
My plugs are coated with automotive 2-part primer and/or paint, and there has been absolutely no damege due to this "treatment".

Regards,
Magne