Martin-style Non-adjustable Truss Rod

I like these in my classical guitars as they are incredibly stiff and not too heavy.

Sometime in the 1970's or 1980's Martin changed to an adjustable truss rod. Prior to that they used a non-adjustable support. After taking apart and repairing a few vintage Martins, I grew to like the idea of the non-adjustable truss rod. Using them I find that the neck under tension gives just about the perfect amount of relief. After all, if a neck gets into trouble due to the neck set, the solution really is not a truss rod adjustment, it's a neck reset, which many old Martins seem to need. I like the simplicity of the non-adjustable rod, it's rigidity and the Martin tradition.
I wish Stewmac offered the older style t-bar non-adjustable truss rod. Maybe they will in the future.

I'm building another Cigar box guitar and this time I would like to try the non adjustable truss rod, because it simplifies some of the operations on the neck.

I'm surprised Stew-Mac even sells this kind of thing. A non adjustable tube that isn't even that stiff when compared to carbon fiber?