I am using Waterbox 20 as well. I use CPR hang-on-back for chatoes refugium. Much more room for growing chatoes than using the tank back chamber which is too narrow and deep. Grow light will be hard to penetrate and not efficient if using the back chamber.
See the link for my a bit old set up. I replaced the AquaMaxx NF-1 Nano skimmer (shown in the photos) with a AquaMaxx HOB 1.5. I will take some new photos if I get the time.
I place Chemi-pure, Purigen, activated carbon in the first chamber with a 3"x3" filter floss pad on top. 1/2 LB of bio balls in the middle section; return pump and pump for h.o.b refugium in the third chamber.
https://www.reef2reef.com/threads/my-diy-wall-mount-for-radion-xr15-g5.735077/