Hi! This is really fascinating stuff, can you elaborate any further on the custom PCB?

I've been investigating it, it's seems incredibly difficult to do?

For instance, I've been looking at a base Arduino + Bluetooth + accelerometer board, but in a custom shape, you seem to have something similar...

Hi! Thanks for your kind words. It's been fairly difficult to pack everything we needed on such small PCB indeed. However we made it with 4-layered PCB and of-the-shelf BT module, IMU with gyroscope and accelerometer and other standard components.