The next task for the box was to cut the grooves where the panel will slide in. The only somewhat appropriate tool I had for this task was my Millers Falls #67 router plane. This is more or less a copy of the Stanley router plane, but without the fence. Unfortunately, the fence is what I really needed. So I decided to make one. The first attempt was just a piece of wood attached to the bottom through the hole in the plane sole. That didn’t work very well.
I decided to get a little more serious about this, and made a combination shoe/fence out of masonite and a strip of yellow-poplar.

This was a nice excuse to use my overly expensive countersink to keep the brass screw heads below the various mating surfaces.

It’s still not the easiest tool in the world to use; to cut a groove, you must move the adjusting nut down between passes while keeping the blade in the same lateral position. You can accomplish this by doing the adjustment while keeping the blade inside the groove you’re in the process of cutting.
There’s just one bit that you can’t get with the shoe/fence attached, and that’s the very end of the stopped groove on the pinboards where you start the cut. There’s a “swimming pool-like” recess there that you need to cut deeper (see the first photo above at the bottom right). To get to that, just remove the shoe/fence and cut in the opposite direction. You don’t need the fence for this because the shallow groove that you already cut guides your blade.

So now I have the four sides and we’re nearly ready for assembly. It’s probably time to complete that bottom panel.