I describe an old project in which a few friends and I designed an RFID tag that fits into a magnetic stir bar and measures temperature wirelessly. We decided to open-source the project, and you can see the PCB design and tag firmware at the github repo below. I don't think I have the firmware for the base station, unfortunately.
https://www.ti.com/product/TMS37157
https://www.digikey.com/en/products/detail/texas-instruments/EZ430-TMS37157/2183733
https://github.com/benkrasnow/Temperature_sensing_stirbar
https://www.patreon.com/AppliedScience