What material is used to make engine covers for passenger airliners?
There are potentially two answers to this question 

If you are asking about engine covers used to keep debris out when the aircraft is parked like these



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They are made out of any manner of fabric, nylon, canvas and often foam filled if they are inserted somewhere. 

If you are asking about the exterior housing around an engine its generally aluminum like the rest of the aircraft. Much like the open panels that can be seen here. 



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