The Chitauri helmet prop had to be created from scratch because all the Chitauri soldiers seen in The Avengers (2012) were purely computer-generated.
Skye mentions The Dude from The Big Lebowski (1998) who is played by Jeff Bridges. Jeff Bridges also played Obadiah Stane/Iron Monger in Iron Man (2008) which is part of the same universe as Agents of S.H.I.E.L.D.