I struggled with this very same question and finally came up with what worked best for my usage. I've included a link to a pictorial web site that I had thrown together for someone that had the same saw as I, and wanted to see how I had built my outfeed table. You are welcome to look at it if you wish to see how I lined up my miter slots using a Festool guide rail and Micro Fence's attachment for my router. Sorry about the extra fluff, but just skip over the pictures that don't pertain to this subject.
http://www.datasystemsltd.com/extension