Here are my opinions.
I have a 29 biocube. Made the first chamber a holder for the Oceanic skimmer. Second chamber a small drip tray with a phosphate pad and carbon pd. Under that a bag of Purigen and a bag of Chemipure Elite. In the rest of the chamber the Stealth heater and a ball of Chaeto with an Azoo palm light over the top. 3rd chamber return pump and ATO float switches. Here is a pic.
In my opinion live rock rubble in the back chamber is a waste as there is not enough of it to really do any good and it would be a trap for any detritus and hard to keep clean.