Before I say anything, beautiful sump!! I had to stop looking because my wallet was in my hand before I knew it..
I think you have two options. You can place it in the sock compartment (My example
https://www.instagram.com/p/BcTXec6jveG/ ). Just take the sock out and place it in there. The other location is between compartment 3 (refugium) and 4 (return section). Looks like there is an overflow from 3 and then goes down through that sponge into 4. You can place it on top of the sponge. Just like that sponge, clean it to ensure good flow, just make sure nothing huge is clumping on it to avoid overflow as with all items. Just my thoughts, but very nice, I'm going back to see your tank, I never made it past the sump, lol....