I rebuilt my PP monitor shroud using black foam board. I think it was about 1/16" thick. You can cut a line with a razor knife, and then bend the material. I only used it as a backer to support the sagging cardboard, but it worked very well.
I bought one for my 27" flat tube and it was about $40 shipped. And then I discovered that it didn't fit correctly and had to be cut down to size in some dimensions and have additional material added in others.