A PC fan is not going to have the cfm required to pull through a carbon scrubber. That box sounds just about the same size as mine that I use for my mother. A 75 or 80cfm fan is going to be plenty for that, but to pull through a scrubber you should get a centrifugal fan instead of an axial fan. Stealth Hydro along with other hydro sites sell the inline fans that you are looking for. Anyone that you find at lowes or home depot etc is going to be an axial fan and not be strong enough to pull through the filter.
Hope that helps brother.