Size of MMM without case?

[mistercrash]

I don't know all the intricacies that go in making a passive heatsink but that doesn't stop me of trying to think of what might work for the MMM ESC. Opinions of someone who knows a lot about heatsinks would be welcomed. I made this with the eMachine Shop software. The base is 2.5 mm thick and it is roughly the same size as the MMM's heatsink at 30 mm by 40 mm. (not exact measurements 'cause I didn't feel like opening my MMM, I just went by eye) What is better between an aluminum heatsink and a copper one?