This is Pat Warner's design. It's a fixture that you clamp to the end of the board, and then use a bearing-guided rabbeting bit to cut a tenon into the stock. Mine is extra-jumbo, beacuse I have some 9-1/2" wide rails to tenon. I totally hated routing MDF, the dust is obnoxious beyond anything I've ever encountered.
The top is facing you in the pic. You clamp the board to the body to keep it registered, and the window in the top gives the cutter access al around the board. To keep the router stable on the top of the fixture, there's a wide acrylic baseplate I made for the plunge router:
This reminded me of the old days, working with acrylic. But back then I didn't have a table saw or a router, so it was a lot easier now that I have a full shop. I need to get some acrylic to work on.
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