This repo contains the production compiled firmware, bootloader and schematic for the single-channel Wifi relay sold via the OpenEnergyMonitor shop.
http://shop.openenergymonitor.com/wifi-mqtt-relay-thermostat/
https://guide.openenergymonitor.org/integrations/mqtt-relay/
Unfortnatly the source code and CAD design is not available for this unit. However the unit is very similar to the open-source Three Channel MQTT Relay that we have adapted to work in single channel mode.