If you're only concerned about biological filtration, you can accomplish that fairly easily by putting a bunch of bioballs or similar media in a sealed container (a large plastic jar, perhaps) and pumping water through it with a fountain pump. You can buy a filter box built for this purpose (with a coarse foam filter) at Lowe's for ~$30. You could also look at that design and build your own for $10-20, less if you've already got the filter foam.
Where do you plan on overwintering the fish? If it's in the container, outdoors, you're likely to have issues with the entire thing freezing solid. In the ground it won't freeze too thick even in very cold weather, but containers freeze from all sides instead of just the top. You do have to consider gas exchange though - the fish can die if there is no way for CO2 to get out and O2 to get in.