if you allready have the fan then your best bet is to just buy your self a carbon filter there like £20 dude which is allmost what you will spend on buying + building it
just match the size of the fan to the filter they start at 4in and go up from there.
edit] dont forget you will need ductin as well
but if you ever get some cash £100 get you every thing including fan/lead bracket carbon filter and ductin + shippin to ya house