I have issue with radiator covers in general! Why do it!!!
Radiators, unlike there name emit 80% or more of their heat through convection (not radiation) so when you box it in the air currents passing around and behind the rad (where the fins are) are reduced or completely
stopped in their tracks.......
Proper installation, rad under the window, this sets up a current that the air is travelling down as it hits the cold window (cooling the air, hot air rises, cold air falls) as the cooler air passes around and behind the rad it warms and rises as it travel across the room, giving a comfortable zone where people are (floor to mid room hieght) then as it reaches the full height of the room and travels as its cooling across the ceiling and hits the window again to restart the cycle.......
At the very least the box cover reduce this and the rads do not work as intended (not to mention your thermostats)