I used a 4" to 6" A/C duct conversion piece. The 6" will go over the 120mm when bent around it. I used expanding insulation foam to fill the gaps. You have to be very careful and go slowly so as not to get any on the fan blades!!!
made this carbon filter with (i wrote up my build here search for it)
construction paper
zip ties
fiberglass door screen
i just place the filter on top of the fan.
its small and compact.
works ok but not great.
If I had to do it again and had the space.
I would use a speed controller + mixed flow fan + carbon filter.
Save the $s and heat issues.
When you upgrade your space (and You WILL), bigger fan is nice to have.