I wouldn't use any flammable coating on the wood, in fact I'd prime it and paint it with fire proof/retardant paint to keep fire down / minimum until it's put out.
If you didn't use any screws I'd also look into putting some in there to keep it from contracting/expanding as the temp and moisture levels change down there. This will prevent the nails from working there way out and it becoming less and less secure over time.
If you didn't use any screws I'd also look into putting some in there to keep it from contracting/expanding as the temp and moisture levels change down there. This will prevent the nails from working there way out and it becoming less and less secure over time.