1) Tolkien's illustration of the moonlit Doors of Durin. Source.
2) Canon tables from the Lindisfarne Gospels. Medieval bibles often included canon tables to show where particular stories were to be found in the four Gospels. Conventionally an archway was drawn around the table, and Tolkien the medieval scholar would have been familiar with the design. The canon tables, which combine a gateway motif with directions into the Gospels, may have inspired the form of the Doors of Durin. Source.
But Jason Fisher makes a good case that the Doors of Durin are a parokhet here.