I have one of them [art. 303] under the bottom bracket for exactly that purpose. This hose holder thing might work well too, perhaps with a small cable tie rather than the plastic bit: https://framebuildersupply.com/collections/braze-ons/products/housing-guides-with-clips-2-10-packs
Yep, those look like it! On closer inspection of Skülly's image above, there's actually one on there - Art. 200 Cable Tie Guide.
@Thrustvector started
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I have one of them [art. 303] under the bottom bracket for exactly that purpose. This hose holder thing might work well too, perhaps with a small cable tie rather than the plastic bit: https://framebuildersupply.com/collections/braze-ons/products/housing-guides-with-clips-2-10-packs