This X axis belt holder is for a specific Prusa i3 variant - ANet A8.
This inexpensive variant is in general solid out of the box, with only a few problems. A weak X axis belt holder is one of them. This prevents you from tensioning X belt enough.
My design is inspired by this thing: http://www.thingiverse.com/thing:926794
The design is available at OnShape here so that you can customize it to your specific size/need.