What size, location, purpose and use for it do you have? Could be the beed sheet if the use is in a localized way, could be a bunch of soft or hard flats along the wall, could be a
scrim or sewed together pieces of fabric inside or without a stretched frame. Could be lots of other things also. Is it after all a full 30' plus by 20' high application with nothing in front of it, some area above the box set that's lit perhaps with some windows showing thru, is it lots of other things down to just a sign of infinity of perspective on the design with lots of stuff
down stage of it and blocking it. What's the back wall look like anyway? Is it red brick, black walls, white with some doorways to merely cover? Is it textured brick or block or flat wall space? Lots of details in description yet to inform of.
If you have
erosion cloth or scrim in front of the
cyc for instance what you need upstage of it is going to possibly be slightly different than a cyc on an open stage.