Wow. I think magnetizing the rectangular hatch on the back was a little overboard (I usually just let that rest, if you're not overturning your rhino, it shouldn't ever fall off), but your method for doing the razorback turret itself is brilliant. For some reason, I never thought of having a free spinning anchor magnet on the underside of the turrets.
For the cupolas, I normally use larger magnets, glue a chunk of sprue on the underside similar to your free spinning anchor, and just glue a magnet to the center of the cupola top, though I usually have to shave down the cupola for that to actually work.
As my cupolas wouldn't be compatible with your method, I can't use that, but consider the razorback turret idea stolen.