Why don’t you look at putting in a trigger systems crystal sumps it’s 18in by 18in, it should fit and it has room for a fuge and a skimmer sectionI'm in the same boat as the OP. I'd like to add a refugium with DSB and grow algae to remove nutrients, but my sump isn't really big enough or set up for it. Was thinking of adding a 20-30 gallon rubbermaid, directing the overflows into that, and bulk heading it to the main sump.
Haven't seen anything like that, and I keep coming across conflicting accounts of proper location in the chain of water flow (before/after skimmer/return pump/etc).