Good idea. Here are the part numbers:

51123417077 Diffusor with a hole for hitch, about $50
51123414413 Cover for the hole, about $20
51127070204 Clamps to hold the cover (2 needed), about $1

I need to check what those prices are for me here... I bet they are a lot more... But that is an easy way of making a clean installation.

- p