I spent a couple of afternoons making the Morley Mortiser for cutting mortises with the router to go with loose tenons. I haven't done any real projects with it yet but did a number of test runs and I am pretty pleased with it so far. It just clamps to your work bench when you are using it.
It cost about $60.00, $35 for a 5 x 5 sheet of 3/4 baltic birch (most of which is left over) and $20.00 in hardware. I had the clamps for holding you work piece already so I saved some there.
Instead of me explaining its use, I have posted Phillip's YouTube video. His plans are $9.95