On my 87 300SDL and the K1500 TD Blazer I use to use a thin laminated board or polystyrene sheet to block off half the radiator in extreme cold weather. It did help get the interior heat going quicker on short trips around town...but on the highway the engines had no problem warming up on their own.
I would not use cardboard....it just turns into a soggy mess from the slush, rain and snow and then you have a paper mache radiator (thats my stupid past experience!). Also be very careful that you do not move any of the core fins with what ever you are stuffing in there...that is a formula for a leak and a replacement of a radiator. If you notice on the trucks and buses they have a tailored cover that snaps onto the outside of the grill and it does not make contact with the radiator for that reason.
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FRED
Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K