Print the two halves and connect them with an elastic cord as shown in the photo. Depending on your printer's extrusion specifics you might have to play with the x/y size compensation to get the parts to mate properly. You can use the dovetail ridges in the box sides to connect multiple boxes together. If they are too difficult to slide together you might need to use a small negative value for x/y size compensation. If they are not tight enough and slide apart by themselves, you don't necessary have to. You can just heat the edge of a couple of the ridges a little and press in a notch into it with your thumbnail (see photo 2). I found this makes it rather easy to get the amount of friction just about right.