This is a bit of an old picture, but you can see the "shiplap" walls there, it is working OK. We ended up having to put in drywall anyway because 1/4" wood isn't very strong... duh. The hole in the wall there is for storage, I will need to build some slide in door to insulate that when we aren't accessing it.
It has been a bit of a dragged out process and is taking way too long. It is tough to find time. The sides are insulated now, but the end wall has yet to be build. So it is really cold up there.