This is the handiest jig I ever built. From ShopNotes. Doesn't even care about thickness of the piece that fits in the dado/rabbet. Just squeeze the piece or sample between the guides, route 1 way and then the other. It's self clamping and the clamping doesn't change the setup, so you make multiple dados after setting the first. See it here.
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http://www.shopnotes.com/main/sn76-toc.html[/url]