Pretty sure you can just make a software implementation on an Arduino or some microcontroller, and connect it to the SPI bus. Of course, it would be relatively easy to extract the keys from an Arduino's flash memory, which undermines the whole concept. An attacker stealing your desktop shouldn't be able to access anything stored in the TPM.

It's like a USB stick, but more secure, and for just a tiny bit of data. A Yubikey is some kind of TPM. In principle, it cannot be cloned if it falls into the wrong hands. Anybody is welcome to build their own, and they are kind of dumb storage elements (that can also encrypt data you send them), it is just not easy to extract data from them.

The firmware for the TPM(+a few functions more) chip in Chromebooks is Open Source. There was a presentation on it in 2018 whose slides can be found on https://web.archive.org/web/20211120005203/https://2018.osfc...

You'd have to adapt it to the SoC you're running it on though. Have fun.