I purchased a new queensize flannel sheet for $2.50 at Goodwill. I plan to add several grommetts to one edge, probably the top, then I will add inconspicuous hooks (probably those you can easily remove) to one wall close to the ceiling to hang the sheet. This will allow me to fold it and put it away when not in use.
Our Extension Office has two 2" thick 7' x 3" pieces styrofoam covered with fabric but they have a huge storage room