halacha - What qualities are necessary in a halachic wrapping?

There are occasions, e.g. a mezuza in a sometimes-unbecoming environment, in which we're required to cover a holy object with one or two wrappings to separate it from various environmental elements. What are the necessary qualities of these wrappings?

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  Do they need to be opaque? If so how opaque? (At the extreme, would clear plastic work?)

  
  



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  Do they need to be watertight or airtight, or can they be porous? If the latter, how large can the holes be? (At the extreme, would a loose net bag work?)

  
  


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  Do they need to be durable? (At the extreme, would tissue paper work?)