Welcome to Border Oak - Border Oak - oak framed houses, oak framed garages and structures.
I think wood beams, faux or real, would help warm up the addition and make it feel more original.
By the way, I discovered Copper Darkening Solution (copper sulfate) on this project - really great stuff. All the hardware is new galvani.
(CJ: Not comfortable seating tho.) Here's a video tutorial that shows you how to make your own inexpensive DIY outdoor bench using a few concrete blocks and some wood beams.
wood casing around opening, white walls, gray painted furniture piece