I'm building a bike with internal cable routing. The inner shift cables run inside the BB shell, under a carbon fiber "shelf". That shelf is does not have any grooves or guides for the cables. And unlike on other frames, the cables do not exit the BB and then reenter further back. There is an access cover that makes it possible to see the cables running under that CF shelf.
My question: should I be installing a cable guide of some sort? The area is rather tight, and I don't see how a cable guide would fit in there. Alternatively, should I use a section of tubing to prevent the cables from cutting into the CF shelf? What is common practice in such a case (this is new to me...)
Finally, the front derailleur cable exits through a hole in the seat tube, without any cable guide. Is there any risk with that cable sawing into the frame?

Thanks for any guidance...

Unfortunately, I just found out that I am not allowed to post images yet.

You could thread a piece of the liner used in shift cable housing over the wires, or there is Teflon tubing referred to as "spaghetti" which is used in the electronics industry which would also serve.