Biggest printer requires 2 carriadge. And the belts no longer cope.
I made the mount for nuts based on the Prusa Mendel. There need M8 threaded rod, 2 nuts and a spring. It really works well.
The top bearing 608zz. And, of course lm8uu.
I have a printable area of 500 × 500 mm. In the original version of axis z the distance between the shafts (axis) 60 mm. I increased it to 120 mm.
see also the set with the distance between the rods is 60 mm. All this and much more is in the group http://www.thingiverse.com/groups/smartrap