So you already decided. Why make a thread?
The only logical place for the drain is the 3rd chamber, at the bottom. Then it's up to you if you want a return over the false wall or back through the back of the 3rd chamber and through the false wall. Anywhere else and you would lose your circulation through the chambers.
I ran a BC14 for years with no skimmer or sump. I never kept SPS, but for softies the rear chambers provide all the filtration and nutrient export you need.
Good luck!