Summary

My LM8UUs were sloppy on the drill rod so rather than trying to find another set of linear bearings with less slop, I just doubled them up.

With them in a pair on each rod, they have very little backlash (if not none).

I'm using KÃ¼hling's (awesome) holders again, but the ones he designed for the X prusa ends.

Instructions

What I used in Slic3r:
0.35 nozzle.
0.20 layer height with 0.22 for the first layer.
45 deg fill at 40 percent fill using line method.
No brim and no support material.

This was done in 3mm ABS on my Prusa Mendel v2.

To use the scad file, you will need the files from the github:
http://github.com/jcrocholl/rostock
and scad files from http://www.thingiverse.com/thing:18384
lm8uu-holder-slim_double-vertical.scad
jonaskuehling-default.scad

The LM8UU bearings should fit snugly in the holders without the bolts and should not require much torque to secure them well. Over tightening may cause binding so slide them up and down when you tighten them to ensure free movement. I made the pair on the the same side to be out of phase with each other by 45 degrees so that the ball tracks do not line up to ensure less backlash (illustrated).

Double Vertical carriage for Rostock with KÃ¼hling's LM8UU holder