You'll need 2X 8mm M3 screws here...
Tapping these holes isn't necessary, but you can if you like.
Just make sure that the screw is flush on both sides so that it doesn't get in the way of the u-joint as it pivots or the dowel as it sits flat in the socket.
More 2X 8mm M3 screws...
Secure the other side of the u-joint to the eye plate.
Make sure the screws are flush... you'll want the eyes (speakers) to sit flat on the base.
Now, go ahead and finish assembling the u-joint.
Wiggle the joint a bit as you add the screws to make sure that the joint doesn't bind this direction either.
Cut a dowel about 120mm long and mix-up some 5-minute epoxy. Glue the dowel into the mount connected to the eye-plate... and let it stand for at least 20-30 minutes.