In my personal opinion, you don't really ~need~ a filter for a single fish. I know a lot of people use them in every tank, right down to the one gallons, but I don't like them, and don't think they are really needed, especially if you keep live plants.
I have a 10 gallon and two 3 gallons, all unfiltered with live plants. The water stays crystal clear (except for that week I had a weird bacterial bloom in the 10 and it looked horrid). I do a 25% on the 10 once a week, and twice a week for the 3's.
I just set up a 20 gallon that does have a filter, because I need something to oxygenate the water and make up for the bioload for the tetras I'm going to put in there. But I wish I didn't need one!
If you really want to use the filter, I'd definitely try the sponge baffle. You could also try a water bottle, but you'd need to get creative with it to make it stay... The sponges are cheap enough that if it doesn't work, you won't have lost much on it.