Once the panels have been cut out it is time to cut and affix the battens. Firstly cut a length of the batten that is the width of the first side piece PLUS twice the thickness of the batten. So for my box this is 24" + 44mm.
Carefully align the batten so that it overlaps the panel by the thickness of the batten at either end (22mm) and the thickness of the plywood at the top (9mm), mark the position of the panel on the batten and apply the glue.
The batten can now be clamped or screwed into position. If you are gong to use screws then I would suggest drilling a pilot hole for each screw before applying the glue. If you really want to do this properly, buy a combination bit for your screws that cuts a pilot hole, clearance hole and countersink all in one go. I use Evostick wood glue and spring clamps.
The side battens are cut next. I cut these to be the same length as the depth of the box, in this case 12". Two of these are required and the overlap the sides of the panel by the width of the batten (22mm) and line up with the edges of the top batten. These battens overhang at the bottom forming the legs.
Measure the distance between the side battens at the bottom of the panel and cut another batten to fit exactly. Cut a piece of wood that is wider than the thickness of the plywood and the same length as the length of the bottom panel between the cut off corners. this will form a rest for the bottom panel. Glue the rest to the bottom batten.
When the glue holding the rest to the bottom batten is set, glue the whole thing to the side panel.
Finally for this first side, measure the distance between the top and bottom battens at the middle of the panel and cut a batten to fit. Glue or screw this into position.