Not sure about the design. Will the media be fully submerged, with water flowing up through the media? If so, that's no good for bio balls or scrubbies. They should be exposed to air with water dripping over them for max surface area for bacteria growth. That design could work with ceramic media, which works fully submerged.
Also, how will the media be secured? I'd put it on an elevate platform, like eggcrate, so it's not just sitting on the bottom, and the water can freely move underneath it, and up.