I used pre-finished hickory. It provides a warm, rich color to the room.

How did you finish out the walls? Framing, insulation, drywall?

Metal studs, spray foam insulation and sheetrock. The tricky part is affixing the stud/rails to the container where it joins the ceiling. I used 1/2" rigid foam and 1/2" plywood for the sub-floor. Clean and seal the container floor with epoxy prior to doing anything - it's loaded with toxic stuff!