Its been some time since I designed this filter and don't have access to my notes at the time as I am traveling out of the country for the next several weeks. The first step in designing any filter is to determine what the objective(s) are. Do you want modest airflow and quiet operation, maximum filtering in a short period of time (Greater Airflow required), etc.
Beyond that there are several fan designs, some move a lot of air at low static pressure (PC computers are good examples) and some filters present a high resistance to airflow and therefore a fan with high static pressure is required.
This design combines a carbon pile and a HEPA filter and requires a high static pressure to get any reasonable airflow.
There are a lot of good fluid dynamics primers on the web that can guide you toward a design that fulfills your needs. In the end you will likely resort to some level of trial and error varying the fan design since the pile and HEPA filter are a given.