Flame retartdant nonconductive panel cover
Sheet Metal
That's really the only choice. That was almost certainly what was there before.
Conductive is actually NOT a problem. If it is designed and implemented correctly, the metal will not be in contact with ANY wires under normal conditions. There is, obviously, a concern if somehow the metal is in contact with one of the hot wires and then someone touches it. The solution to that problem is related to the other concern about a panel cover - access. Basically, you want access to be possible (a) without tools and (b) without contact should that be a concern. An ordinary breaker panel will have a door that on hinges that can be opened/closed easily - that is essentially what you want to build here. If it is done right, you will, if needed, be able to open/close with an insulated item (e.g., a dry wooden pole) to access the breakers. But the cover itself metal. Probably some sort of steel, though aluminum may be more corrosion resistant.