Things to notice:
1. Currently only items are addable.

2. Do not set the delay to a small value when you have a large number of boxes because it will cause the server to lag.

Command usage:
/boxes add-loc
Adds the current location to box spawn locations.

/boxes add-reward [iron/gold/diamond]
Adds the item in your hand to the list of the specified box items.

/boxes add-preview [iron/gold/diamond]
Adds a box with GUI of prizes (max viewable items: 36)

Chances:
Iron: 60% Gold: 30% Diamond: 10%

Configuration:
setup: 0
set to 1 to reset values to defaults.

small: [true/false]
true - the head sizes and stands will be small.

sound: [true/false]
true - plays a sound when the boxes are opened.

message: [true/false]
true - sends a message when the boxes are opened.

delay: 7
the number of seconds before the next box is spawned.

iron-texture/gold-texture/diamond-texture: the base64 value of the head that you want the box to have.