It was an almost trivial design to knock together in OpenSCAD. I've rendered it in 1 mm increments from 1 to 10 mm, but if you want a custom thickness, change one number and re-render.

If you have an Anet A8 with the injection-molded X-axis motor mount, try the 3-mm spacer. Replace the M3x20 countersunk screws with M3x25 button-, pan-, or cap-head screws. It'll get the X-axis motor good and snug, whereas following the directions leaves it kinda loose in the mount.

Update (7 Jan 19): needed a spacer for the extruder on the Hypercube 300 I'm building, and decided to revisit and clean up my first-ever design. I'm just a bit more proficient with OpenSCAD now. :)

Also on Thingiverse.