Half square plastic tube. Where to find?

[Alfons_Helikopter]

Sounds a lot like a channel for electric cables, like Thorsman. Check out the electrical wholesalers' sites and the electrical departments at the building supply stores.

 

[Mikael_L]

Mja, I think there is (or at least has been) a kind of skirting board combined as a cable channel. I put it up in a room in the mid-90s.
You would attach brackets at regular intervals on the walls, and in these brackets, you could place several cables "separated" from each other.
Then you would finally snap on the cable channel/skirting board, and as far as I remember, it was empty of moldings inside...

 

[Mats]

Real thorsman were at least the last time I worked with them completely sealed on three sides. They were attached just as mentioned with special brackets on the backside.