To strengthen my necks, and to prevent future warping I install a carbon fiber beam down the center of the neck. The beam I normally use is 1/8 x 3/8 inches. I route a 1/8 inch channel for the beam when I make up the neck blank. This 1/8 inch slot becomes a reference for the center of the neck on a number of subsequent jigs that have 1/8 inch pins on them. The slot fits over the pins and allow the neck to slide back and forth to fit the jig. The carbon fiber is glued in only rather late in the neck making process, when all the jig-based cutting is done.
For baritones which have longer necks, and parlor guitars, I was thinking that I would like to add a bit larger carbon fiber beam, but that 1/8 inch slot was integral to my build process. I realized that I could cut the 1/8 inch slot and use it as the jig reference right up to the point that I would normally install the carbon. Right at that point I could put the neck back on the router table and widen the slot. For a baritone I got some carbon fiber which is 3/16 thick, a bit more robust that the 1/8 inch. For a parlor guitar I could make the slot 1/4 inch and just double the 1/8 inch carbon.
The carbon fits tightly into the slots, I have to tap it into place. I used to glue them in with epoxy but epoxy is messy, and the carbon fits so tightly most of the epoxy got squeezed out making more of a mess. Now I just tap in the carbon and flood it with thin CA glue which wicks in and glues everything together with no surface mess to clean up.
