EDIT: This design is defunct. See my other 'thing' titled Mendel Upper Z supports. I'm leaving this old design up here for now for reasons of comparison.
Do Not Download and print as-is. These are for reference only.
One big design problem with the prusa mendel I2, is the entire weight of the carriage assembly hangs on the Z-motors and whatever couplers you use. The aluminum barrels will stretch, and the plastic clamps and plastic tube idea will creep. Gravity is a patient beast, pulling downwards forever and never tiring.
I use the plastic couplers, and can attest from experience that things will creep downward. I lost about 1mm every two weeks and found myself adjusting things often. I grew tired of that.
These are my solution. They transfer the weight from the rods through a skate bearing down to the threaded rod below, taking all weight off the couplers.
You WILL need a 3D modeler and some basic skill to make use of these. You will need to measure the distance from the ends of your Z rods down to the threaded rod below and adjust the models. You will also need to measure the distance for the standoff support to the other rod and make adjustments.
One other thing. I drilled a hole through near the base and use a single zip tie to secure it to the rod. You can model in a hole or drill one post print, but I'd recommend it. Compression should hold it in place and the threads will keep it from moving, but a little insurance doesn't hurt.