Using a box-slot bit to form grooves in the box sides for the bottom saves you the trouble of filling in gaps at the corners, made by cutting the grooves on the tablesaw or router table with a straight bit.

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CornerRadii

Using a box-slot bit to form grooves in the box sides for the bottom saves you the trouble of filling in gaps at the corners, made by cutting the grooves on the tablesaw or router table with a straight bit. The trade-off is that you must round the corners of the bottom to match the radius of the slot bit. Here's a simple way to quickly form perfect corner radii:

Chuck a round-over bit that matches the radius of the slot cutter into your table-mounted router (516 " for this bit). Position the fence flush with the bit pilot bearing and parallel to the miter-gauge slot. Then attach an extension to the miter gauge with the end just grazing the face of the fence. Now, with the bottom (B) against the fence and backed by the extension, rout the corners, as shown above.