Main guy of Onekey hardware wallet, called Yishi, created nice little guide to help people making your own DIY signing devices aka hardware wallets.
There is nothing new written in this guide, but it might help someone who is concerned with recent hacks and exploits on branded hardware wallets.
Devices in this guide are based on Raspberry Pi and ESP32 boards, and I already mentioned them in Seedsigner, Krux, etc.
https://diywallet.org/