As long as the grave that is dug has a burial vault in the grave for the body then there's no requirement for the body to be buried inside a casket.
You can be buried with or without a casket and it's your choice entirely.
My Grandpa was buried without a casket on his own land and only had to have a concrete burial vault which is poured himself before he died.
He decided that he didn't want a casket and would rather just be buried directly in the burial vault that he made himself.
So you can be buried with or without a casket if you want too and there's no law that says you must be buried in a casket.
There's also no law that says that a casket has to be made of a certain material or construction.
You could be buried in a wood box casket or anything else and even cardboard.