The concern about plastic was flamability. You can select such tiles that are not. This is why I do NOT recommend the plastic idea. Heat from the luminaire, radiating could be enought to ignite the plastic. What if one of your bats of insulation falls on the plastic and the weight causes one of your staples to pull loose and drape directly on the exposed bulb? instant fire. Plastic is NOT a good idea. One might search for fire-retardant chemical imbedded so-rated canvas-like material, but that's a much higher investment and loses its rating every place you invade it with a nail/staple. IF you have a light fixture there or anywhere, I wouldn't recommend plastic. Secondly evaporation through the pores of the basement floor and walls will through moisture into the air. This moisture now wicks through the fiberglass batts, your floor through your home and out through the attic/walls, etc. plastic will seal that moisture in your basement, and that office and basement in general will acquire a musty/moldy/mildewy situation in no-time flat. Not a good idea. You want a dust retaining, flame retardant, BREATHABLE cover there -- that's why I recommended the tile system, just make sure its appropriately rated.