I've assembled and tested a different than OTGW hardware for opentherm boiler control.
Shop and additional info available here: Domoticz OpenTherm Thermostat.
They has a few other integration samples as well as gateway or OT slave setup, so it can coexist with already installed wall units.
For just a thermostat or boiler remote controlling build is pretty straightforward, you just need to connect OT shield to the boiler and flash example sketch into ESP8266/ESP32 board.
Connection overview:

Skect is on github repo.
That product is quite new and authors are maintaining it, so we can request some features.