Well lets think about this for a minute......................
The thermostat is 190deg(approx.) which means it shouldn't be open at all unless coolant at the thermostat is 190deg
So there would be no, or very little, coolant flowing to radiator until t-stat opens a bit
So the covering of the radiator with card board is not done for the radiator, its done to prevent cold air from coming into engine bay while driving
Radiator should stay cold in winter months, its not a required part of the engine's coolant circulating system, its only used to bleed off EXTRA HEAT generated by the engine, thats the point of the t-stat, it only includes the radiator IF engine coolant gets above 190degF
If temp is still low that means the engine itself can not generate enough heat with fan circulating the cooler air in the engine bay
You could remove the fan but thats a pain, an electric fan conversion is a possibility but costs money
Check your fan clutch, it may be broken and stuck in "locked" mode
After engine sits over night or at least 6 hours, open the hood and try to spin fan, it should be hard to spin
Start cold engine, let it run for 20-30seconds, shut it off
Try to spin fan again, should be EASY to spin, unlocked mode, if not then fan clutch is bad, which causes too much air circulation in cold weather
You could insulate the heater hoses, doesn't cost much for those "pool noddle" type foam insulation tubes
Heater hoses do bleed alot of heat, even a bit of insulation on the longer sections can help
Feel free to cover the whole radiator to stop the air flow into engine bay
You see that done on many semi trucks in winter, they use zippered or snap on grill covers to stop air flow into engine bay to keep engine temps up