alan_m wrote:

20mm black (or white) electrical conduit pipe appears strong enough to
remain vertical without additional internal supports and its around 50p
per metre for single pipe lengths (comes in 2 or 3 metre lengths).

Where the black conduit has been fixed to walls, without leaving a
suitable expansion gap at joints and junction boxes, I've seen it warp
horribly in hot weather, whether than means it gets sufficiently soft
that it can't support its own weight I don't know ...