Cube Shell - Black (8 ct.) - Rules of Play says:

Boost your cube with Dragon Shield Cube Shells™. Cube Shells™ are small deck boxes in a strong material with a firm lock. Cube Shells™ are ideal as booster packs for Cube drafting your favorite card games, boxes for sideboards, side decks or extra decks, or just to sort and order cards in your board games. Sending multiple card in the mail? Cube Shells™ offer strong and sturdy protection during shipment.