I suppose placing some fiberglass ceiling tiles or foam in there would be a good idea. What's the screen made of? Does it sound particularly reflective by itself? My guess is that the screen\absorber combo might reflect real high frequnecies back and absorb lower ones, to a degree.
So I suppose this wouldn't really be so much a "diffusor" as the subject header says, but rather an absorber. But hey, absorbers in bare, hardwooded areas are always a good thing!