Surely the long term answer must lie with an alternate thermostat. As Rog says, it's totally bonkers to correct an overheating issue by reducing the efficiency of the rad.

Thermostats respond to a heat dynamic, a blanking plate can't.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone