Just get the little things wiredog posted a link to. Simple, cheap and effective.
This seems like the way to go.
You will need to trim the edges, and those are more to prevent water from getting in than air from coming out. The air is either coming from the header in the attick, or from the expansion groves in the brick. Either way, you could stop the air at the point of entry and fix your problem. You may have too much return in your AC, if the supply is restricted, it will make the area negitive. This will cause your house to bring in air any where it can get it. If you fix it at the switch, where will it draw from next?