The plans I found recommend stiffening the travel guitar’s neck with carbon fiber. This makes sense because it lightens the neck slightly by substituting carbon for maple and because the travel guitar will want to have as thin a neck as possible without being weakened. This guitar will also use heavy jazz strings so some added stiffening will help the truss rod.
Stewart MacDonald carries these. The plans suggested 3/16×1/4×18.
I decided to orient the rod with long section dimension vertical to stiffen more in the direction of the string pull. It was placed by eye to evenly distribute the truss rod (not yet installed) and the stiffeners (2).
I used the table saw to cut the 3/16 by 1/4 deep slot. The Stanley plow plane cleaned up the slot. I made the fit “not snug” so I would get at least some glue line.
West System 3 was used with the long time one hour hardener in this warm weather.
Only one neck was stiffened. The other neck will now be set aside as a spare.
-- Dave, New England - “We are made to persist. that's how we find out who we are.” ― Tobias Wolff