PEX is of course cross linked PE, the main advantage is high temp or chemical resistance, not much advantage in this application? It the cable really 1/4? or 6mm? If 6mm you could use 1/4 ID, if its really 1/4 you probably need to find 7mm or 5/16 ID. PVC and maybe polypropylene would probably work well too. I think the enemy is mostly going to be UV. PEEK would be great, stiffer and much stronger than PE, but the price will make a strong man faint…and then we get to Torlon and Vespel and Ultem at national dept type pricing.

McMaster has a pretty good selection of these in different colors. I would stick with opaque to keep the algae from growing inside.