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.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 ;-)