Thanks for the response.
I believe the suggestion you made is what I described as "offset walls"; I guess that's just what I'll have to do, then. I found an example in the SU 3D Warehouse, this pic is a screenshot:
Rather than extend the left section (green in my pic above), I'd want to squeeze the walls of the right section (purple) closer together. I guess I didn't realize this type of feature needed to be designed around in this way.
I do plan to submit my drawing to an engineer after I finish, they'll probably have to redraw the entire thing. Prior to that, though, I intend to get it front of somebody with some experience, to get some idea what I did right and wrong. I'll learn a lot more from critique than I will if I just hire it out.