If you would like to mask the cutting line in order to not see the core, you can use CNC cut strips (0.5-1 mm thick and lets say 8 mm width) of carbon fiber sheets or U profiles and glue them to mask the core (see attachment).
In order to make a box you can use cut and hold method, it means that you can cut honeyomb sheet skin only from 1 side, then bend the sheet and glue the cutting line. See some photos attached. I hope it will help you.
You can make some reinforcements with L profiles glued in the corners of the boxes (inside).
You can use some inserts to screw the box panels together (see photos) .
It can be used with skins made from glass or carbon fiber sheets.