It's become apparent that flexible aluminum shaft couplers are not the best way to go for holding the Z axis threaded rods. The deal is, they don't flex in the directions we could use (side to side) but they do flex up and down with the weight of the extruder (not good). To make matters worse, vibrations cause them to bounce around and in some cases this becomes noticeable as z-wobble on vertical surfaces of prints (granted, it is subtle). So, I've started using a printed shaft coupler that I designed, and I wanted to share some info about it today. This is obviously not the first printable shaft coupler design. In fact, my first reprap printer (a Mendel i2) used printed shaft couplers. I remember because they were a pain in the butt to assemble and adjust. This one is a single piece, and uses a double-shell design that gives it a nice amount of flex/compression while still being pretty resistant to cracking (as always, I'm printing them in PLA).
It's located in the stl folder on the Wilson2 repo (https://github.com/mjrice/Wilson2/blob/master/stl/coupler-5x8.stl).
Assembly is straight-forward: fit two M3x10mm screws through the holes and loosely-attach a hex nut to each one (note that one side has insets for the hex nut and one is smooth for the head of the screw). Thread the 8mm lead screw through the top side until it bottoms out (there are two little "nubs" that are meant to engage the threads, so you can't just slide the lead screw into place). Next, slide the coupler over the top of your motor shaft (it's ok to let the threaded rod hit the top of the motor shaft). Then just tighten down the screws nice and snug and you're all set!