Which good conductor is not a solid material?
Aluminum is a good conductor because of it's atomic structure, that has nothing to do with it's geometry. Fins certainly make it much more efficient but it's not like it stops transferring heat from hot side to cold side just because the it...
2 things are happening, the aluminum is still transmitting heat to the cooler surrounding air via convection and it's giving off heat via radiation (hopefully more than it's reflecting). It's very good at both.
What do you think is happening?
Aluminum is a great conductor, that's why it's used a heat sink for so many widgets. I wouldn't worry to much about big sheets of aluminum trapping too much heat. By adding aluminum skids, you're basically adding huge heat sinks with a massive blower (air from your velocity) over them.