This is the toughest part of the project. It is also nail dependent. The dimensions shown are for nails I had available in my shop (see step 2). If you have a different size, adjust the holes according. As I mentioned in the intro, mixing 3D printing with wood working is an option. I've designed and included STEP and STL files for printable versions of these parts. The countersink mounting holes also allow you to adjust the placement in the box.

For the woodworkers, I started with a large piece of hardwood scrap. The thickness was 1/8" thicker than twice what was required for the parts. This worked out perfectly since I wanted to drill the holes once and the cut the piece in half. The extra 1/8" accounted for the blade width. I drilled the holes as shown - see first picture. You could also add the tapered (countersinks) at this point as well - slow down and think while doing that step. Next, cut the block to 1/2" wide by 3.5" long. Now cut the block along the centerline to make two pieces. Make sure to mark the pieces so you can reinstall them with matching holes.

Drill and countersink holes on the ends as shown. Note that these holes are for # 6 flat head screws.