Place an Imitation Window centered below the arch. Sink it slightly into the partition using the trotting glitch (walk into it while placing).
Behind the partition, place a Glade Lantern or Candlelight Chandelier to make the window glow. 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
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