This design will replace the awful stock X axis gantry on the One-Up. No more sag ! Plus, you'll win 2-3 centimeters of printable surface.
The holes may need a bit drilling, it's a tight fit, in order to eliminate the sag. You'll need a M10 screw for the belt tension system, and some M3 screws for the motor. (2x14mm and 2x8mm should be good)
I recommend you to print the left part upside down, that way it'll need less support material.
If it is well calibrated you can print this with the stock One-Up !
This design should work for a two-up too, please let me know. Also, let me now if there is anything wrong.
NOTE: the carriage used is derivated from my model here: http://www.thingiverse.com/thing:1488156
IMPORTANT EDIT: the carriage is made for a GT2 belt :)
The carriage does not need support ! And, if your bridging game is strong, neither do the right part.