Update: I finally cut some of my plastic sheeting to size, and inserted it. I cut it very carefully, so that the ends butt together with a little pressure; it seems very secure on its own, without any adhesive. And, they're quiet, so I'll just see how long they last, or stay in place. Worst case, it falls out and I'm back where I started.
Scotty, I don't know if this shroud is made to be used alone, but there really isn't much room between the shroud and spring. My plastic sheet is only .020" thick, and I can feel the difference in the clearance when I insert it.