There is a point underneath the top two bearing retainers on the Di3 extruder assembly where the X-axis belt rubs when the printer is in operation. Below is the STL for an "Extruder Skate" that attaches to the rear of the retainers and creates a buffer so that the belt will not wear as fast. The ends of the skate are tapered upwards to allow the belt an un-obstructed contact point.
We recommend slicing using the "solid" setting in your slicer so as to retain strength. You should also have your filament path move along the long length of the skate for minimal obstruction.
The hex screw sizes are M3 x 25mm and the nuts are size M3.
The arrow below shows the point of contact that creates the problem:
The Extruder Skate:
The nut that secures the skate in place: