I made this for a friend's birthday, a mechanical numberpad with a standard layout and one encoder. The case is printed, with options for a single-color case, or the multicolor ladybug logo. It's powered by a KB2040 microcontroller from Adafruit soldered directly to the board without headers. The PCB cost about $20 total including for five boards at a PCB fab (this was several months ago), to order one you only need to provide them the Gerbers.zip file. I used store-bought keycaps, but any will do.

With the included QMK firmware, the encoder adjusts the volume. I tried to make it so clicking the encoder button switches to layer 2 where the knob controls the screen brightness, but I couldn't get it working.

I've attached the KiCad PCB project, the Inskcape .svg for the PCB art, the QMK  config files, and the Blender file for the case. Feel free to remix however you'd like!