Mate you should look at replacing your fridge seals before anything else.
Extra insulation is largely useless unless what is already there is airtight. The major cause of lost efficiency is humid air entering the fridge. The fridge then is forced to condense the water vapour before it can cool the air further. What I understand you are planning to do is stick on some extra foam to your fridge, but still open and close it the normal way. My professional opinion is that this will have very little effect, and in fact may be detrimental if you cover areas that need to radiate heat (some fridges place the condenser coil behind the exterior cladding).
I would strongly recommend against you doing this. Not because it won't theoretically work, but because the payback on your investment is going to be longer than the rest of your life, and it will make your fridge look a bit silly.
Otherwise I'd be going to Clark Rubber or the like if you want Styrofoam. Typically, insulation on refrige systems uses metal cladding mounted - in this case to the outside of your fridge - and then the void between is filled with expanding foam.
