The GM parts manual calls them a “Cover” and sometimes a “Shield”. I just bought a new one for an 2006 8.1L that cost around $48. They have clips in the back the cover pops into and some years of engine use a center bolt towards the front to hold it down. Later engines use a large rubber ball that screws into the intake that the cover pops onto. The screw ball thing is about $2.
What year is your engine? Or, what year of cover do you want? There are a few different ones for Chevrolet small blocks, Cadillac, Pontiac, etc.