mold in place with posterboard and packing tape
I did just as shipchief did, I cut off and scarfed the existing skirt about mid length. Then, I taped on a strip of posterboard where I wanted it, covered it with packing tape (which makes a great mold release). then I waxed it, and layed up three plys of 8 oz glass cloth. Then, with it all still in place, I rough sanded it and tried to trim the edges some, careful not to scratch the aluminum skins too bad.
Then, I spread on a medium-stiff mix of microballoons (with a little bit of cab-o-sil to prevent saging). I sanded it to contour, covering and fairing out the various waves and defects in the initial lay-up, and at the same time, made it about 1/16"+ thick everywhere except along the top and bottom edges. Then, I layed up two more plys of glass on top of that.
The microballoon layer makes a sandwich construction that results in a very stiff skirt, as well as creating a nice smooth, fair shape for the final plies of glass, so less fill/finish work later. The finish/fill was done with a somewhat wet layer of microballoons and cabosil spread very thin, sand smooth, etc.
To me, this was way easier than wrestling with trying to get a good fit on the provided pieces.
