samb is right, you absolutely want to seal the compartment like your life depends on it.
Since you're so high up off the floor and you plan to have two panels, you may not generate enough chimney effect to make the refer run efficiently. You'll find that a number of people have improved efficiency by adding 12v
computer fans to help the exhaust along. They are placed high in the exhaust vent. With a switch, you can control when you run them. If I remember correctly, they are needed more in hot weather, which makes perfect sense.
If you want more of a chimney effect, you can use one of the refrigerator vents like this
. It also requires a roof vent, which is hard to find in your vintage, but can be made to look really good. With the taller stack, you shouldn't need fans.