...and there I was thinking you meant to submerge the lamps
Round here, I tend to use solid off‑white PVC which obviously won't rust, or you could try solid brass/aluminium etc. Unfortunately rust‑resistant materials can still attract condensation and dust and moulds. Under certain circumstances the copper in the cables will conduct heat away above the ceiling so the light fitting will be slightly cooler than the rest of the room and that may attract the condensation. A metal frame being a better conductor of heat than PVC may worsen that effect. The simpler and smoother the shape of the fitting, the easier it will be to clean and the less trouble you will have with mould.