I worked a lot with composites in my last job. Carbon fiber, just like fiberglass cloth, provide directional strength, depending on the weave (or lack thereof). A straight flat sheet of fibers running the length of the neck parallel to the face would provide side to side warpage resistance, but do nothing for top to bottom. 90 degree crossed fibers will provide resistance to it cupping across the width as well as side to side, but will still allow top to bottom flexing of the neck over the longest dimension.
In essence, the fibers, be it carbon or glass need to be parallel across the direction of stress, so to prevent flexing of the neck along the length, the fibers would have to be perpendicular (stacked from the top face to the bottom face and running the length of it.) Kind of like running a steel spline down the center of it through the thickness.
If the back of the neck is curved, you could stiffen it some by wrapping the cloth around the radius of the curve on the back, (basically giving you a half tube),.
To relate this to wood:
Think of a thin piece of pine 4' wide cut along the grain. It will bow and cup easily, but because of the width, does not warp much side to side. Take two pieces and glue them together with the grain running perpendicular (i.e. plywood). It still bows and cups but there is no warp across the flat dimension. It stays square. The more layers you add, the stiffer it gets. Add a piece glued on edge vertical to the surface, and it stops the bow. Add another piece vertical to the first but perpendicular to the second, and you also stop the cup (ie torsion box). Same principal applies to fiberglass and composites. With Fiberglass, you also have matte mold, or short sections of fiber sprayed into the resin at all angles. This is more like particle board or MDF..
Not sure on what "stiffening" you are trying to achieve.
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