Here is a video that was shot at Comic Con with Mark Hamill saying that he does not remember ever filming that scene when he was constructing his new green lightsaber in the cave.
Looks like George Lucas may have used a stand-in for this one....or did he?
The strange thing about this whole video interview is that is was labeled as being taped from 2009, but the clip was just released.
Very strange and my take on this is that I don't care either way because it is still a cool video.