Heat Sink for portable refrigerator
No, don't think that works.

In order to see what happens, I plugged some numbers in here. The calculater doesn't support a flat plate, so I used only two 'fins' (the minimum).
Here are the numbers that I used, sort of modeling the long side of you box:

H - 1
L - 200
W - 70
b - 0.5
t - 1
N - 2
T - 25 (ambient air)
natural convection, vertical base
Emissivity - 0.09 (from here)
Ls - 70
Ws - 70
P - 75W  

The resulting source temperature is a bit above 650C - not good...

Thermal resistance of the plate is about 6.6C/W, which means that for every watt you dissipate, the plate's temperature will rise 6.6C.