Summary

The easiest way to stabilise the z-axis is to connect them with a rod. More stiffness you will reach with two rods. In combination with a additional clamp between the rods will generate a rigid frame.

Instructions

Before you fasten the clamps make sure that the z-rods have the same distance on top as on the base. With out this stabilisation the z-rods are free to align to the x-ends. If the stabilisation frame is mounted the z-rods are fixed and can't move anymore.
While fasten the clamps the x-ends have to be on the highest possible position to get the exact distance.

If your Printrbot is long, you can take more of the "double clamp middle" between the ends.

Attention:
Control if there is a collision between the frame and anything of the x-unit. If so, edit the z-max position otherwise you will have now a very robust unyielding mechanical endstop ;-)

To assemble you need:
4 srews M3 x 20 mm with nuts and washers
8 screws M3 x 12 mm with nuts and washers
2 smooth rods 8 mm as long as you need

Now the frame looks a bit like a handle but even though never use it to lift your Printrbot ;-)

Z-Axis Double Rod Stabilizer for Printrbot