Whatever works! I ended up covering about 1/4 of the rad with cardboard and it helped. I didn't want to cover too much in case we get a few warmer days this winter.
A word of warning that you might want to insure you cover the entire radiator in front of the fan. I know that some fan blades can fracture due to the fan blades flexing as they load and unload due to "partial" covering of the fan blade area.
Tim