The WSTL18 temperature logger is slightly larger than a CR2032 coin-cell battery. With most of the Atmega328PB's pins broken out to headers around the board's perimeter, it doubles as a coin-cell development board.

To make expansion shield making easier, I've made a template shield with all pins labeled. The entire board plus shield will fit in a readily available 5ml acrylic jar, as long as common-sized components are used. For anything larger, such as a LoRa module or LCD screen, you'll have to find another casing solution.

The shield includes a milled opening not seen in this rendering, to allow access to the uart connector pads on the WSTL18 below.

The shield template is available in Eagle format on the project's Github (go to Eagle -> WSTL18_SHIELD).