In terms of design, smart home sensors are usually simple, unobtrusive devices and I wanted to keep it that way by using a white, 3D-printed case. Since the device can function as a contact sensor, it had to be small enough to be attached to a window or door frame. To figure out the necessary dimensions, I first searched for a suitable battery holder. Many common AAA holders consist of a plastic tray with two contacts, such as the Keystone 1020. However, since the case will be 3D-printed anyway, the whole plastic tray would take up unnecessary space. I ended up choosing the Keystone 56, which uses individual SMT-solderable spring contacts that add almost no length or height to the AAA cell. These sheet metal contacts are inexpensive and available from multiple manufacturers.

With two of these contacts mounted to one side of a PCB, the other side could be fully used for the electronic components. This means that the overall size of the device is defined by the AAA cell, the battery holders and a standard 1.6mm PCB. There was just one more important consideration: the antenna. It needs to stick out from under the battery to work efficiently. To keep the cost and size down, I opted for a quarter-wave trace antenna which adds around 6 mm to the PCB length. The negative antenna terminal sits right next to the antenna and becomes part of the RF ground.

The case mainly consists of two snap-fit shells that can be opened to replace the battery. Inside the front shell, there is another 3D-printed piece for holding the PCB in place and guiding the battery to its contacts. This inlay piece is also essential to ensure that the AAA cell can only be inserted the correct way. This is achieved by recessing the positive terminal slightly. 
Zigbee Devices require some mechanism to start the pairing process, so there is a hole in the case for a pushbutton. This button is attached to the case via a small hinge. By changing the filament mid-print, the button gains a transparent window to the status led beneath it.

Another mini-version of the main device houses the magnets for contact sensing. I am using a set of six 2x5mm magnets, which seem to be strong enough to trigger the Hall effect switch. 

Both the sensor and the magnet case can be mounted to a window or door frame using double-sided tape. When it’s time to replace the battery, the main part of the case can simply be pulled off.