What to do with the "Black Hole"?

[Andrew Faukner]

We are going to rehab an A-frame cabin in the mountains in California. The problem is that the ceiling has a "black hole" left over from when a fireplace stovepipe was removed. When the owner put on a new metal roof, the roofers just put up tar paper, then the metal. There is no attic in this type of structure. The wall/ceiling looks like it is a plywood with grooves, almost like a heavy duty paneling. I don't know if I can buy more of this type of plywood. The cabin is about 60 years old. The hole is about 3 feet tall, by about 1 foot wide. While I just wanted to stick up some glow-in-the-dark stars to make it appear like you were looking out into space, my wife said, "NO!". Any ideas how to patch, or cover up this black hole?