I imagine you could purchase these activated charcoal sheets that you can cut to size! Tape a plate with a hole over it and you should be able to cut any size you want. The fan you spec'd looks like it should be able to handle the load that the charcoal filter will cause (it is going to be like trying to breathe through a straw rather than just your mouth).

Another option would be something like this where you have an all in one filter/blower system. It's more money but it will handle all that obstruction better. Alternatively you can go to a harware store and buy an inline duct booster. You should be able to control the fan speed with a big triac or possibly PWM signal.