Lockbox

Lockboxes are lockable containers that can be placed on a Housing plot for storage or decoration. A lockbox can hold up to 30 different stacks of items and a maximum of 2000 stones (weight). When crafted they have a key inside that can be used to lock the box.

Source
Lockboxes can be crafted through Carpentry with 55 skill points and the following materials: Lockboxes become trivial to make at 80 skill points.
 * 4 Wooden Boards