I thought I would share a technique with you which I picked up from a great builder who is not a member CBN. I think you can argue that a 3 or 4 string CBG doesn't really need any additional neck strength, but I've found myself making thinner necks recently, and I'd like to make sure they stand the test of time. I am also gearing up to have a go at making a 6 string from scratch so this is a technique I will need to learn sooner or later.
So, to do this you will need some mild steel bar. I got 3x13mm. A 4 metre length cost me about £4. The postage cost twice that but it was almost worth it to see the look on the delivery driver's face when he handed over a 4m long cardboard tube. You also need a router, and a bit of the correct width and depth for the bar.
I used the parallel guide on the router to slide along the side of the neck to cut the channel. I used a high speed and 4 shallow passes to get the finished depth (which needs to be a tiny bit deeper that the width of the bar)