This is floor stander, right? Lock miters are a strong way to build cabinets, but this may be difficult, if not impossible, to get the precision needed for a slide-in back that long. What do you intend on removable top/bottom so back can be slipped up/down. I'd also be concerned about the panel rattling, if I made joints loose enough to easily slide. Is it a sealed system?
I admire your creativity, but it really looks like reinventing the wheel to me. If you gotta have access other than driver holes, why not a screw-on panel on back? And if you're gonna be tweeking a bunch, perhaps a outboard crossover.
FWIW, I've attached a pic of back panel of a speaker I built... was sure I'd wanna have access so I meticulously put in machine screws with t-nuts. In seven or so years I've had them off exactly.....never.
