If the rabbet will be on the inside, a router table and a rabbeting bit with a bearing would do this easily, but you will need to finish the corners with a chisel. To cut the rabbet you just lay the box down over the bit and let the bearing on the bit guide you as you slide the box around the bit while it cuts the rabbet.
I frequently build boxes and use this same technique with a small diameter slotting bit to cut a slot for the box bottom. I clamp the sides of the box together without glue, usually using a band clamp, for this step. Small diameter bearing guided slotting bits are available from Lee Valley for doing this. I then cut the box bottom to the proper size to fit in the slots and round the corners slightly to allow for the corner radius left by the slot cutter in the corners. With the small diameter slotting bits, this radius is very small so it is hidden when the box is assembled.
Central North Carolina