I was looking at some ancient corner filters I adopted from my dad probably from the 70s. He used to put marbles under the floss, just to weight them down, and the unit acted as a simple mechanical filter. But with a different weighted substrate that has more surface area, they could now function primarily (or at least equally) as bio filters. But forget the fancy plastic box, just grab the nearest jar! The beauty of this approach is that one can easily match the filter size with the bioload by using a different size jar with more or less substrate and airflow. These are done with a 1-qt mason jar (but you can use any size you want/need/have) and an old cartridge housing from an RO filter after the threads broke off. Just drop your biomedia in the bottom, floss on top, and a 1/2" PVC pipe down the middle--just right for the airstone. Cheap, easy, and made from stuff I already have sitting around--perfect DIY. Enjoy.
