This is essentially exactly as described in an old issue of Woodsmith (and available from Woodsmith plans). I chose it because I liked the way it allows you to switch between different sizes of fingers without having to set up the jig each time (after initial setup). I use it with the freud 1/4-3/8 box joint blade.

Here's the working end of the jig and you can also see the replaceable zero clearance insert:

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Here's the back end where you can see how the sides slip into the miter slots to guide the jig. Cutting the base block so the sides fit snugly in the miter slots with no slop is the fussiest part of the build, but wasn't really a problem. You can also see the knobs that hold the interchangeable front fences to the main part of the jig:

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This shot shows how the interchangeable fences accommodate multiple finger sizes without requiring setup each time. The screw on the end of each fence acts as a stop to position the key in the proper position. Once a fence is dialed in for a particular size, the stop allows removing and replacing the fence in the exact same position. I made a fence assembly for 1/4 and one for 3/8 fingers since those are the sizes I mostly use. The jig can handle any size from 1/8 to 3/4. While I was at it, I made extra fence assemblies and extra zero clearance inserts in case I decide to use a different size down the road.

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Here's a bottom view showing how the sides extend to act as runners and how the base block acts as a blade guard:

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I milled up some 6 inch wide 1/4" thick stock and went through the setup procedure for 1/4 and 3/8 fingers. The 3/8 sub-fence was dead-on on the first try. The 1/4 version was a bit too tight so it took 2 tries to get it dialed in. I've got several small projects lined up to make with the jig, so I'm looking forward to putting it to use.