I have a 5G. I use an aquarium sponge attached with an elastic to the front of the place where the water spills out of the filter.
I do something almost Identical to this. I have live plants so I don't need the filter for collecting debris, I just use mine for water circulation. So I took all the filter media out and placed the aq sponge inside. It takes at least 80% of my water flow down. So I highly suggest going with Canuck's advice. Eventually though if it's a rubber band it'll rot and snap (ending up inside your fish tank, bad idea in the long run but good for short use). I'd probably use some thread to tie it on or fishing line instead of the rubber band.
It's labelled as "Aquarium Foam" in pet stores, but it's just a fish safe sponge really. I'd get the bigger ones labeled for larger aquariums (I got one meant for 150-200 gallon tanks) because they seem denser and cut back water better.