Recently, I was able to view the Parement of Narbonne in person, and noticed a detail that wasn't visible in reproductions available to me earlier. A lantern is shown in the panel showing the betrayal of Jesus. In a good reproduction, you can see that each pane is divided by a horizontal line. In the same scene on p. 181 of the Tres Belles Heures, each pane is divided by a double horizontal line.
Viewing the Parement closely it is possible to see a small pair of circles below the horizontal line on each pane, as though each pane was made of two pieces of horn riveted together.
This site has a useful 15th c. view of a similar lantern from the 15th c. Lyversberg Passion. In that lantern the seam appears to be covered on the outside with a metal strip, with the heads of rivets visible on the inside of the pane. Note the lace or thong for opening the door.
Note also the importance of making a door wide enough to allow comfortable access to replace candles: the Tres Belles Heures lantern seems to use eight vertical panes (each composed of two pieces of horn joined together.) The door is two panes wide. The lantern in the Lyversberg Passion seems to use half as many panes, so a door using a single pane is practical.