I would scronge up some Tyvek house wrap and use that inside the box.
It's micro pore breathable fabric would keep the soil from contacting the wood along with it's wood destroying organisms, but allow the moisture to wick away. A double wrap should work and the Tyvek is inherently stable with no chemical leaching.
It's micro pore breathable fabric would keep the soil from contacting the wood along with it's wood destroying organisms, but allow the moisture to wick away. A double wrap should work and the Tyvek is inherently stable with no chemical leaching.