To strengthen the miter joints in the corners of a box like this one, I add splines. The splines create additional gluing surface and help prevent the miter joints from opening up over time. The hardwood splines are glued into slots cut across the miter joints. The easiest way to cut these slots is at the table saw.
The trick is holding the box at a 45° angle while cutting the slots. To do this, I made a simple jig like the one shown here. The jig is just a short fence with a couple of supports that cradle the box at the proper angle.
The jig rides against the rip fence of your table saw. This lets you use the rip fence to locate the slots on the box. After you set the rip fence for the first slot, simply rotate the box to cut identical slots on all four corners before moving on to the next slot.
