If you're wanting to split a 20 long into 3 chambers to allow for a refugium, a Reef Octopus NWB series is definitely out. I've got a 150, and it's pretty big.... it'd take up nearly half of your sump alone. I'd probably recommend one of Reef Octopus HOB
skimmers to allow for the maximum amount of room in your sump.
As I was writing the above, I figured since it was time to change the filter sock, and I had an old 20 long sitting around, here's some perspective shots of how large the NWB150 is sitting in it.... By the time you put two sets of spaced baffles in there, not much room left for a center refugium.