It's a great idea. I've been meaning to build one for a while. It's on my long list of "I'll get around to it, eventually" projects.
Guy who created youtube channel
Crypto Guide has a bunch of videos about DIY Trezor and other DIY signing devices and hardware wallets.
He created DIY PiTrezor in several different shapes, using different hardware and he is always posting links with instructions and hardware he uses.
I think he even made Terzor using STM32 Dev Boards in different video, so it is worth checking out his older videos also.