Place an Imitation Window centered below the arch. Sink it slightly into the partition using the trotting glitch (walk into it while placing).
Use Stage Panels rotated vertically to create a thin stone or wood frame on the left and right sides of the arch. simple arched window ffxiv
Glitch two Wood Slat Partitions sideways (using the floating glitch: place on wall, rotate, cancel placement) to form the top curve. Angle them slightly inward at the top. Place an Imitation Window centered below the arch
Behind the partition, place a Glade Lantern or Candlelight Chandelier to make the window glow. simple arched window ffxiv