This website has a link to a few "DIY" hardware wallets based on microcontrollers:
https://diybitcoinhardware.com/ They also have links to some libraries that might be of use to you like
a version of secp256k1 for embedded systems. Perhaps a "hardware wallet" is not exactly what you were after, but there should be some crossover between the wallet functionality these devices try to provide and what it appears you are trying to.