I heard that keeping your fridge full allows it to run more efficiently, because the food doesn't transfer the "coolth" to the outside as fast as the air does.
Assuming your standard refrigerator is not full, why not put jugs of air (cools fast) or water (stays cool longer) into the fridge in the empty parts? If you do this, there will be less volumetric air to escape, so the fridge will be able to retain the "coolth" when you open the door.
trebuchet03 
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