What material is the heat sink, copper or Aluminium?

I'm afraid this just won't work for many different reasons.

The mechanics of a laptop don't give you the space to fashion anything to replace what Apple has designed.

Note the fans (2) are blowing across a finned radiator unit which is attached to the heat transfer armature from the heat sink plate that sits onto of the CPU & GPU. This heat sink is designed with a very small clearance for a thin coating of thermo paste.

You would need to mill down very precisely the heat sink in order to make enough room for the Coollaboratory pad which would weaken the structure and lessen the transfer of heat to the armature.

Just so you know Apple did try pads on the GPU's of early iMacs. The issue they faced was boil off of the liquid component used which then made the heat transfer less effective. They quickly went back to the thermo paste design. I don't see how Coollaboratory can get around this problem.