Anthony DiGirolamo has made a great Teensy Thumb Keyboard.
This small tactile keyboard is super to use with handheld projects. It has 60 keys arranged in a QWERTY layout and number keys arranged in a number pad layout. The board has beginner friendly through hole soldering with a Teensy, 60 pushbuttons, and 10 zener diodes.
Anthony used this thumb board to make a handheld Raspberry Pi. Using the Teensy he was able to add backlight control and low batter monitoring in addition to controlling the keyboard.
Code for the project can be found in the Teensy Thumb Keyboard Github repository. KiCad files are available or you can also order the PCB from Tindie. Finally, you can find the files to 3D print the case used for the handheld Pi project on Thingverse.