Machined side and top/bottom plates. These are covered with white copy paper adhered with 2 sided tape, to enhance reflectivity and reduce specular reflections off the aluminum. This adds about 3% edge brightness, to reduce the overall panel gradient to less than 2.5% over the surface.
Aluminum overkill? Definitely. But I have the raw materials and the means to machine it. I don't have the same tools for wood.