Update 11/5/18: Added 1.5mm base to the thick version.Update 11/4/18: Added a thick version (+2mm thickness).Update 3/11/17 : New hole, added fillet.Update 10/23/16 : increased thickness for screw holder and legs.
A simple, adjustable belt tensioner + a belt tie to replace the cable tie.
The tensioner requires an M3 screw. The screw hole is tight-fit and should provide more than enough tension to hold the screw. Designed to be printed at 100% scale.
Use higher wall line count (shells) & high infill for maximum strength.
The calibration cube in 3rd photo was printed @0.1mm layer height with  properly tensioned belts and a Dual Z mod https://www.thingiverse.com/thing:2163949
sketchup/fusion360 file is attached in case adjustment is needed.