This is what I would do with my sump if this was mine!
First chamber is water in from main tank, Here I would have LR and Chaeto/caulpera or of the like with LR and also skimmer!
Next I would add a piece of glass to create a baffle! The baffle is raised around 1 and 1/2 inches to let water into the middle chamber, this will help reduce micro bubbles and in flow and it will have LR rubble in it too. This will also aid in raising water level to create more surface area to clean.
Next is your refugium where you will have dsb around 3-5 inches deep I would but an inch of miracle mud too or buy some live sand from your lfs! this could even be sand from their system!!
Into a new baffle! which again will have LR Rubble in it to aid in filtration, then a final chamber where you will run a return pump,
No need for any carbon pads or nio balls or external filters!
You can buy a HOB overflow/weir from main tank and run it into sump!
Please this is worth a look into! its much much better than any chemical filtration or external filters.
Dont listen to the "its too small" stuff !! you can see it really isnt!
If you are unsure still look at this!!
Sumps
and take a look at mine. If you want more details then measure your sump exactly PM me and I will do the math and tell you exactly what you need!
Cheers Robin