Cut a pice of plastic to shape from an old milk carton or whatever and bolt it in place with the two bolts that support the exhaust bracket, then seal round the edges with silicone. Takes ten minutes. If you can be bothered or you're a perfectionist, make one out of aluminium sheet much like the one you get in the box if you buy the official accessory Akrapovic exhaust.

Here's one person's way of doing it...

http://www.xt660.com/showthread.php?t=17143

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