On the What's On Your Workbench tread there is a discussion on truss rods. Although not normally required on a 3-4 string CBG [if the neck material and dimensions are suitable] there are times when one could be of benefit. As Tom says in the discussion he would fit one as he uses heavy gauge strings, and of course with a six string a rod is the norm.

Any way, photo's should tell the story. I'll admit its not my idea I got it from Cumpiano's book back in  1989. My first choice is factory double action rods, but I still use this design for some builds.

You can see the amount of adjustment is available in the completed rod  photo, of course it would be bending down away from the pull of the strings. The black rubber spacers were applied to restrict vibration it the channel, but not required with a good tight fit.

I cold bend the rod and then hammer the two legs tightly together at the bend.

You may never need a rod but if you do you know how, and its in the spirit of DIY.

Cheers Taff