Couples a 5mm motor shaft to an 8mm threaded rod, ensuring they are coaxial, but allowing some angular movement if the rod is not straight. This version has the advantage of using two identical parts, rather than having a captive-nut side, and a smooth side. This means that one needs one fewer part type, which seems like a good idea, but it also ensures that the bolts balance one another.
As mentioned here: http://things.hands.com/thing/pla_coupling/ this version avoids the use of center=true, which seems to have been causing printing difficulties for some reason (as yet undetermined).
Oh, and it has round corners (it's mostly an intersection of a 'cube' and a cylinder)
Place some plastic tubing over the motor shaft and then clamp it with M3x20 bolts
Alternatively (as mentioned in the comment by Pointedstick) one can reduce the diameter of the smaller radius hole to 2.5mm, and forget about the tubing, but this will of course reduce the suspension element of the design, so will presumably be less tolerant of bent rods.