The interior could have been drywalled in a day, and would have been much cheaper, but I really wanted to have the warmth of a blond wood on the interior to contrast with the black and metal on the outside. I bought a bunch of 4' x 8' 1/4" birch plywood and we cut it down into pieces to create the pattern. Interior panels took two guys 5 days. Ouch.  Amazing Modern Backyard Office by Aaron Britt