Paul, you should be able to figure out the shape of the former fairly easily.
Cut a series of 1/4" square balsa to fit across (or top to bottom) the fuselage in the spot where the former needs to be. Space them about 1/2" apart or so. Make them just long enough to fit between the walls and hold themselves in place without distorting the fiberglass.

Something like this:

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<<>>
<<<>>>
<<<<<>>>>>
<<<<<>>>>>
<<<<<>>>>>
<<<>>>
<<<>>>
<<>>


(the forum software kind of screws up what I was trying to show there, but I think you will get the idea)

When all the sticks are in place you can CA a long stick of balsa at 90 degrees across all of them to lock them together. Then you should be able to pull the entire thing out as one piece, or if you have to you can cut it in half. Trace around the outside of the sticks, then cut out your former.