Follow-up -- inspired by the discussion above, I bought a sheet of egg crate light diffuser from Lowe's yesterday. It cost around $13 for a rather large sheet -- I now have a lifetime's supply! It is easy to cut out pieces to size, using just a pair of sturdy scissors. A sharp knife will trim the "nubbins" left behind.
One thing that surprised me -- I couldn't find this on the Lowe's website when I searched for it, but since I was by there I stopped in on the off chance. It turns out that it is not sold in the lighting section of the store, but rather in the building materials as a type of ceiling panel.