Axial to Inline fan? PLEASE HELP!

Does anyone know how to make an axial fan into an inline duct fan? i've got a 300cfm axial fan that i'd like to hook up to a carbon filter. Any previous posts on this?

Sleepless

Ducting > make ducting fit > fan > make fan fit > carbon filter or more ducting.
This would fit well in a growroom design forum.