I made a box joint jig following [https://www.youtube.com/watch?v=EyJof__nTR4](Woodfather's video). It's a nice simple/flexible design for those of us without a dado stack.
First try was very sloppy, but once I adjusted the key width and got my clamps set up better the fit is great. The scraps I had around were bed slats off the curb, which were very cupped. But they actually turned out pretty nicely (after plenty of cleanup).
Boiled linseed oil finish.
Thanks! I want to try some smaller / varying size fingers next.