At long last, I found a short mention of using white foamboard from Office Depot (Staples, whatever). Desperate, I went with that suggestion and was very pleasantly surprised with the results. I bought several 2′ x 3′ sheets of white foamboard – five in total. The foamboard’s ultra-white surface has the wonderful ability to gently disperse light. The foamboard is also very rigid and makes it easy to construct a 5 sided box that doesn’t get wobbly or weak.