What are the cable-management panels made from?

They are black powder-coated steel rather than a plastic cable-management moulding.

Depending on the model, the main sheet is approximately 1.3 to 1.5mm thick.

That makes them reassuringly prosaic pieces of rack hardware: there is very little here to flex, sag or become an elaborate contrivance.

On the ring versions, the steel rings form a substantial part of the assembly and are permanently attached.