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The BID fiber orientation should be 45 degrees for strength.

Take a layup of 4-bid (45 deg orientation(say 3 inches by 6) and bend it...
twist it, etc.  This is easy.  Take a 3 in by 6 in piece of 3/8, 2 bid per
side, foam or honeycomb filled piece of material(45 deg orientation) and do
the same test.  This twisting and bending is resisted to a much greater
extent.  The "web" depth between the two "skins" proportionally affects the
stiffness.  Maybe you want to add 1 BID to each side of the 1/4 in prepreg
material and see how that compares to the 3/8 material.

When trying to transfer stress concentrations, it is always best to taper the
layers of glass (look at the spar to fuselage transfer pads as an example).
So, the decreasing size of the original successive glass layers is not
equivalent to making a thicker bar of narrower width using the same amount of
glass.  The stress on the fuselage would be concentrated at the point where
all the layers ended rather than being spread over a larger area.  

Using just glass alone is the heaviest way to achieve stiffness.  

A suggestion for your consideration:
Micro 1/2 inch close fitting foam to the "joggle" channel.  Spread a coat of
micro on the top and fill the holes where the foam meets the side walls of the
channel. Add bid (45 deg) on top per original suggestion (perhaps not quite so
wide).

Before doing structural glass work, read Burt Rutan material (the real
expert).

Scott Krueger
N92EX