Quoted from zene10:Caskets and skulls are metal.
If anyone is interested in a coffin mod this one looks cool as heck. I talked to zene10 and he said right now he isn't making them but there has been a couple people wanting his mod and he said if he can get 10-15 people wanting it he will make them. He is starting a list so if you would like one please pm him and get on the list