A natural covering (as by a fluid)
Else clues• Academy Awards prop
• Award presenter's prop
• Awards show request
• Greek goddess of mail?
• It gets opened before some speeches
• It might have a window
• Letter holder
• Mail holder
• Stationery enclosure
• Wrapper, of sorts
• The bag containing the gas in a balloon
• A flat rectangular paper container for papers
• Any wrapper or covering
• A curve that is tangent to each of a family of curves
• The maximum operating capability of a system