Yeah that pic only shows half of the sump, there are 3 sections. The only thing I can come up with is like you said. I was going to go buy a couple of huge bags of poly-fill from the craft store... and stuff it into the 1st section around the drain pipes that would kind of force the water to go through it to get to the next section.
A sump is not the filter... Its used for housing/hiding all the equipment needed for reef tanks usually, But I am setting this one up for freshwater for the time being. So with that said I am trying to figure out a way to make it into a filter. So my adding mechanical filter material such as poly fill or filter floss to filter the the debris in the water moving through the sump is my objective. Make since?