Previous version
It was akward, looked ugly. To adjust belt tension of one axis, it requires to loose the vertex frame then turn the belt adjuster bolt. The problem is when the vertex frame is moves up, it affects alignment of the whole frame.
New version
Source: http://www.thingiverse.com/make:208879
Parts:
M3x30 (I used M3x35) x1
M3x15 x2
F623ZZ x2
M3 washer x8 (used five washers on the idler)
M3 nut x3
M3x30 bolt x1
M3 nut x1
M3 washer x1
Leftside of the idler bearings – M3 washer x3
Rightside of the idler bearings – M3 washer x2
M3x15 x2
M3 washer x2
M3 nut x2
Freedom, World peace in unity! 