I've thought about doing that too. Here are my thoughts...
  1. If it's gonna be a simple design, I thought of twisting the inside piece of screen so that it forms a cone.
  2. Then plug the small end with a good grade of silicone.
  3. Repeat the above one more time starting with a much larger piece of screen to form an "outer cone".
  4. This should (in theory) allow one to pour the carbon into the big end just prior to attaching the inlet flange.
  5. Now, temporarily pin the inner cone so that it won't unravel.
  6. Adjust it to be the same size as the inlet diameter on the flange, and pin it together permanently (a few safety pins should work).
  7. Now it would be time to fit the outer cone around the inner cone in such a way that it would allow the gap between the two to be filled with the carbon.
  8. Adjust the outer cone like ya did the inner cone with one exception. The outer cone should obviously be as large as possible while just barely allowing you to squeeze the flange onto it.
  9. Now that you have a filter element with a flange on one end, attach a piece of pipe long enough to extend past the small end of the cone to the flange.
  10. Attach the "outlet" flange to the piece of pipe and VIOLA'! You have an inline carbon scrubber!
This should do the trick quite nicely. The design is used in other industries to filter other compounds. In most of those that I've seen, sintered bronze was the filtering media used.