Commercial 'Smart Thermostats' bug the hell out of me.

A Thermostat is a temperature sensor, and a relay. Anyone who has a little bit of tinkering experience could probably build a smart thermostat that doesn't need the cloud and is totally private.

However, accepting that most people can't be bother to DIY, or even have the needed skills, I'm surprised there aren't 'Open Hardware' Thermostats available that are just as easy to fit?

I've not Googled, but does anyone know of any non-DIY 'open' alternatives ?