You could do a couple of things.....
First, the stock fan is a low output fan about 30cfm.
You would be better off modding the case like
Buffaloman57 suggests for better air flow.
I have a 7237 case and put in bigger fans to get
good cooling.
If you don't want to do this at least put in a better
fan, a Sunon high output 80mm, in the front
and rear of the case.
As for the filter you could use duct tape and
carefully tape it over the fan hole, or buy
the cheap foam packages for AC window units
and tape it to the inside of your bezel.
Hope this helps....