I used to have refugium in my WB 50.3 AIO. Purchased large IM media basket, removed middle shelves and attached Tunze Eco Chic Submersible Refugium Light.
Long story short, Cheato grew much faster than I expected and stripped my tank of Nitrates. And, since I didn't have any phosphates because dry rock absorbed everything, SCA overtaken my tank. After three months of fighting, I decided to start over.
So, it is easy to set up refugium in AIO filter chamber, but please do check your macro algae frequently and adjust light periods to make sure you reduce your nutrients and not removing them completely.