I vote for a simple small lambs tounge stopping about an inch or so from the corners. The trick of course is finding a router bit with bottom bearing that will fit in the space you have.
Matte black lacquer would be a good finish choice because of its quick drying if you have any experience spraying lacquer.
Matte black lacquer would be a good finish choice because of its quick drying if you have any experience spraying lacquer.