When i got my carbon in the mail some was pellets, some was lil chunks. the different sizes can be used for diff types of carbon, while still using the hepa air filter recommended by jack@meow
The author remixed this model.
At 1st i just added a grid to remove material on bottom, then i found another model that had some ridges to help keep the holes from getting covered, i added this in after repairing them model a bit.