Hardware stores were scoured in search of just the right diffuser material that could be bought ready-made or didn’t require too much extra work or precise cutting. Acrylic? Vinyl? Something higher tech and laser-cut? As it turns out, the easiest solution wasn’t in the hardware store at all, but over at the craft store…
You don’t even need a whole roll like this. It’ll be stretched out, so a piece about 2/3 the length of the bar will suffice.
A few 1" strips were cut from black foam core board (mat board or illustration board can work as well) then glued to the sides of the frame to create a shallow channel. It ain’t rocket surgery!