NEVER fit a radiator muff or blind.
The inevitable outcome will be overheating.
The best way to improve the heater and the engine efficiency by the way, is to fit a hotter thermostat. and I suggest you get one that LOOKS identical to the one you have in the car, BUT is rated to open at 92°C.
The stat will stay shut till it reaches about 90°C, during which time the coolant circulates through the by-pass and the HEATER, and heats up quickly.
As soon as the coolant reaches about 90-92°C the thermostat opens and sends some VERY VERY HOT coolant to the radiator thus dispacing the same volume of cool water into the engine.
If you have the radiator blanked off, it will take only 2 to 10 minutes to overheat the engine depending on driving conditions at the time. The worst case would be up a long steep hill in a low gear towing a big heavy trailer.
CharlesY