These are the next boxes we use at the SOE Compound.
We have used wood and metal in the past. The wood gets nasty and hold the funk. The metal rusts eventually in the humid TN weather. I like the plastic as it can be removes at washed if needed. Not a bad idea to do once a year or so.
We use pine shaving in them or straw. For our coops we use a 10×10 chain link dog kennel and have a pice of wood on the outside and a piece on the inside with the chain link sandwiched in between and we just use 1″ drywall screws to hold these nest boxes in place. We have been using the same boxes for over three years now without issue.