You are correct about where the carbon goes - I used the crystal/pearl type kitty litter myself, much cheaper and easier to get and just (?) as efficient.

The polyfill is like a "cap" on the carbon, just on the ends, so it doesnt move and stays compacted - the center is totally empty and is where you pull air through (or push air into)

Make sure you get metal hardware cloth that is plastic coated, so the pantyhose slides better - you can use 2 on top of each other if you put a run in them

GO to Lane Bryant, Or Catherines, or any local "PLus Size" womans shop, and get the biggest pair of pantyhose they have Size F (5'3"-5'9", 400-450 lb)
.. (Hehe the lady looked at me when I asked and said "Sir, these would be way too big on you!")
They won't rip if you are careful pulling them over the shell, but you can double them up if ya rip em - just costs 2x.. the foot of the panties will be on the bottom, and the top is held up by the hoseclamp