I used Figma to create the star shape which I've exported to Eagle CAD using Gordon Williams' SVG to Eagle CAD converter.
I ordered the PCBs from JLCPCB and assembled them myself. I was wondering if they'd be able to cut the PCB at such sharp angles and I'm actually pleased with how they look.
Here's the top of the PCB:
And here's the back:
I've added 4 pads on the back, in case I want to solder some hooks on it to use it as a wearable. The detailed schematic and the Eagle CAD files are here.
I've placed the ISP pads on the back of the star in order to program the ATtiny using this DIY pogo-pins adaptor for my USBasp programmer (3.3V logic).
I've used the MicroCore ATtiny13A core and (very important!) I've disabled the brown-out detection (BOD) when burning the bootloader. If it's set too high the LEDs won't turn on when the battery starts discharging. The code is also on Github.