First thing to build is the inner chamber.

pic1 - supplies:
6"-4" reducer
4" cap
2 - 4.5" screw fasteners
10" length of Heavy duty chicken wire
2 - pannyhose legs

pic2 - Wrap the 10" length of chicken wire around the 4" end cap to form a cylinder. I used a bunch of zip ties to secure the cylinder's shape.

Pic3 - Slide the pannyhose legs over the cylinder to act as a screen for the carbon.

Pic4 - Secure the end cap on one end of the cylinder with one of the 4.5" screw fasteners.

Pic5 - Secure the 6"-4" reducer to the open end of the cylinder with the other 4.5" screw fastener.

That's it. The inner chamber is done. Set aside we'll need it in a bit.