The drywall panel that covers the back of the electrical panel is quite damaged and broken up. I would like to replace it. The back of the panel is flush with the inside drywall. Do I have to replace it with another piece of drywall, or can I use a sheet of plywood? I would much rather use plywood to cover the panel and the romex. I like the extra strength, so the damage doesn't happen again. Is it code that the panel must have drywall behind it?