I picked up a cascade internal filter w/ adjustable flow up to 45 gph for just $12. That or something similar probably would be a decent store bought idea... In a bigger tank you can attach all sorts of containers but with such a low volume, the bulk of an assembled rig would leave little room to scape plant and leave room for the fish to swim.
As far as DIY's that small, I've never really found anything decent aside from sponge filters.
Okay... $13.. but it has a reusable sponge and an air inlet, you can up the flow by removing the little adjuster thing for the flow.