This is a modified design based on the original printrbot x-axis carriage. It's much stronger and uses four bearings rather than two. It has built in belt attachment/clamp surfaces. It has a wider surface for attaching to the extruder mount (designed for my modified extruder mount).
I have included the sketchup so you can adjust to fit your specific needs or modify it (for example adjusting the tightness of the press-fit holes for the bearings).