The best thing for the Anet A8 to avoid any Z Axis wobble, errors, travel issues, etc is to Straighten Up those wild Z threaded rods! Binding the top, center or any other part of the Z axis will not help as much as making the two rods on each side parallel to one another.
I took Simhopp's Replacement Motor Mount, and I adjusted it so that it has slotted (oval) screw holes for the four motor screws. Also, I adjusted the motor hole so that it is also slotted (oval). Each Z Axis motor can be slid to the left or right. This should straighten up those threaded rods and allow the Z carriages to slide up and down smoothly.
Now, it is possible to reprint the carriages on either side so that they straighten up the Z threaded rod, too. They can still be used with this mod to tweak it just right. And replacing that fragile acrylic is always a good thing (I keep mine as spares).
I measured the center of the threaded rod to the center of the non threaded rod on each side. I now have 23mm on top and bottom for BOTH sides, so I know they are pretty parallel.
I printed this using PLA and 60 infill (They can bear weight if tilting the printer). I pounded the top plate onto each side plate with a rubber mallet, and I screwed them together (tight parts fit = no vibration). Then, each mount box slid right into the frame, no issue, no mallet, no anything. It is an easy replacement, but it took a little bit of fiddling to get the axis perfect straight. Also, I installed a cork gasket over each motor to help with vibration.
Each mount was three separate pieces (rather than one single piece), and this was done for two reasons:
1) Strength of materials - long thin layers on the side walls are more brittle than a large layered area.
2) If your Z Axis is really bad BUT at least can print under about 10mm (probably closer to 8mm), you can print this (still need good X and Y axis).
I am not going to mark this as a work in progress as the initial prints fitted together nicely and worked as expected. It's on my Anet A8 right now.
Here is a link to the motor gaskets so that you know what I am talking about:
The original motor mount:
Also, a big thanks to Simhopp for doing this! Please note, the included side plate is the exact side plate Simhopp had, I just re-added it here for completeness. I like having everything all together.
UPDATE October 10, 2017
Added the other side plate as a mirror to the side plate. Print one "side plate" and one "other side plate" for each motor mount.