One piece X-axis carriage with LM8UU in Heeks
by Wired1, published
These two carriages are a simple way of easily positioning the y carriage on the rails and do away with the need for four separate bearing holders that each need to be mounted and aligned.
You still need a lower carriage on top of this to mount the upper carriage from and then your pcb heater.
The next evolution of this is http://www.thingiverse.com/thing:29744 which does away with all the bits in between the pcb heater and the carriage at the cost of a longer print time.