When I took my Fett pictures to a local seamstress to see about doing a custom softsuit, she said she used to work a lot with the Navy overseas and she said she thought the boots looked a lot like a British Neoprene boot. Now, I have no idea when this "Neoprene" stuff was invented, but she said to check out dive boots of different kinds. I admit I didn't put much stock in this assessment, but maybe it's worth checking out-- she was right about the little blousing strap with hook and loop at the bottom of the softsuit-- also used in these military duds overseas, according to her.
Just a thought.