The most challenging aspect of this project was fitting all parts into a very confined space. I chose a readily available ABS plastic enclosure designed for USB sticks. The available board space inside was meant for one PCB only, but two sides of one PCB weren't enough for all of the components. I knew I had to find a way to stack two PCBs inside.

After thoroughly measuring the available inner dimensions, I knew it could be done, but all components would have to be 2mm or less in height. Digging through many parts catalogs and with inspiration from smartphone board design, I was able to find the board-to-board connectors and the most important part - a micro-SD and nano-SIM two-in-one card holder.

Due to the constraints, some components are slightly more expensive than their bulkier counterparts, but no compromises could be made. I am extremely happy with the final design of the boards.