DIY 3 way refrigerator venting (drawing)

I'd try to get the intake air up from the bottom of chassis and as far away from the fuel filler as possible. Any flammable gas will tend to sink and possibly, pool in a confined space. The last thing you want is to pull flammable vapor into the fridge space.

Old-Biscuit probably has the ventilation nailed. If you install a top-mount vent, a fan probably wouldn't be necessary but many do actively push air to increase efficiency.