Added message 'boxWasDroppedToGround' to player when box was dropped onto ground when player inventory is full. You need to add this message to locale files, if they already exist. Alternatively you can copy this message from default locales in plugin JAR.
Added possibility to specify player head texture in dropped items. To do this, 'item' must be PLAYER_HEAD (SKULL_ITEM for 1.12 and lower), and 'texture' must specify base64 texture value. You can see example in default boxconfig in plugin JAR.
Tested box opening and head textures on 1.8 and 1.15