Heat sink for sure BUT the heat remains IN the room, so after the inital warming to the temperature of the room, it no longer functions as a heat sink.
I also fear aluminum as a grow-bed construction material, as it still corrodes easily if not treated. I was just on the phone with Dusto talking about sacrificial anodes actually! I believe aluminum is a very common material for these- any plumbers in the house who can confirm that? (To test if you are a plumber, we will also need a picture of your ass)