Those covers are usually made a bit bigger, especially if they come with padding inside (velvet etc.), which I actually don’t like.
This one fits to a milimeter, and I’m confident that no grain of dust will ever get in! Also, it’s pretty strong and feels waterproof.
Strongly recommended to any Poly D owner!
PS Please note that this cover is meant to be used only when synth module is in collapsed position. Also, you have to pull out all your cables. This is fine for me, because I have a small desk and I have to move around the Poly when I’m not using it, but might not be right for you.