Why do you say that a screw with a finer thread would create a stronger bond? Wouldn't a finer thread be more liable to get stripped, especially in plastic? In my experience, fine threads are only good for machine screws, whereas coarse threads are best suited for wood, drywall, and plastic. Or does the courseness of the threads really matter anyway, since most of the work is being the done by the nuts clamping it together?
I'm sorry I've never commented before, even though I've been following this thread for months. Your reengineering (hey, it's not often you see two double 'ee's in one word), scratch building, and detail work are just incredible.