How about putting two 2" poret dividers across the sump with appropriate PPI (so 4" total), then fill large portion of the sump with lava rock (assuming that is an ok option) something like 50 gallons worth, and maybe one more poret foam block/wall right before the return pump. Would probably create walls around lava rock out of egg crate (light diffuser stuff from lowes).
Just not sure how useful lava rock would be, how well water flows through it etc.
This over lava rock. Mucho better. You'll need less of it for more bio capacity.