Based on a few different designs I've seen floating around including the commercially available one from Bay City Tools

I printed this at 30% infill and 3 exterior walls and used PLA to avoid the warping since the ends needed to be very square.

Hardware needed: 2qty. M4x20mm hex cap bolts Lowes 2qty. M4 nuts. Lowes

The set up to cut those two very different sized lap joints pictured above took just seconds and there are no gaps.

Here is a very simple step by step on how to use it for some half lap joints. https://stusshed.com/tag/km-1/

And here is a video turorial https://www.youtube.com/watch?v=PQ6_S6lZKLI

EDIT: Added the 123D Design files too incase anyone wants to monkey with the design. Just be sure to "remix" it from mine.

EDIT: I've just completed the Tenonmaker as well, Thing:1354302