My opinion is based on appearance, and this would be a dust cover.
As such, they run from $38 on Ebay on up, like this, and can be found on Marco, and other pin vendor sites.
ebay.com link: Pinball Machine Dust Cover Choose your Size
What is wanted, usually, are the UV Protective covers, at $84.
ebay.com link: Pinskin Dust Cover Protector for Standard Size Pinball Machine
I am awaiting quotes on piggyback orders, to see if its possible to get this lower.