A few years ago I worked at Peavey Electronics, they were working on a feedback removal device. It detected feedback and eliminated it, whatever the frequency. I really didn't do much, I was a co-op at the time and was just going through the steps of testing it. What I did learn though is that nearly every room has frequencies that it will amplify depending on the shape and size of the room. Also, it depends on how far apart your mic and speakers are as to what frequencies will be filtered.
As far as I know, there's no one filter you can make to eliminate feedback, but if your speakers are in a fixed position as well as the mic, then you'll need to come up with a way to find out what frequencies are the most likely to create feedback and filter those. I'm not sure that there is a simple way to do it effectively for little cost. There are however, devices you can buy that will eliminate feedback for you, and they are very simple to use.... I did a LOT of testing on them, but that was years and years ago and I don't remember much.
Most of the other people here are much more knowledgable than I am though, perhaps they could help you more than I could.