On a bike with a top tube brake run, the coaxial cable is heat shrunk to the hose. On another bike which has redundant gear cable fittings, I've put the coaxial cable inside some PU tubing and used those, if you're interested I'll dig out the tubing size, it was a tight fit but the next size up wouldn't work. On a previous bike, I super-glued the coaxial along the underside of the top tube, with a wrap of helicopter tape at either end, that was in place for sixteen years and only came off when the bike was resprayed, at which time removing the cable also removed some paint.
All three are probably over the top and took some time to get right, I've nothing better to do and expect then to last the lifetime of the bike. The part I am sometimes dissatisfied with is from the light to the frame, under the fork crown is neater, but without mudguards probably a bit vulnerable. I'm pleased with the tubing, hidden in plain sight, for a bike that's going to get some abuse (Frame bags rubbing?) it's pretty indestructible and runs from light to light and secured at both ends, I could probably lift the bike by it.