Remixed from a common & effective design, this mod is designed for beginners, who've just recently assembled their kit... I extended the arm by 6mm for more play. The added length may cut into maximum printing width if your carriage hits it, but I made it longer to vastly help with belt placement until a proper belt clamp is printed. Just use an extra M3 30 screw from your kit.
If you already have an effective belt clip mod, and no longer need zip ties to pinch your belt together, this belt tensioner is NOT recommended.
This tension arm remix also works with 8.5x13mm 16 & 20 toothed aluminum idler pulleys.
This is a simplistic design, meant to be used as a disposable work-around. It only uses two small pieces which easily fit into the existing bearing block, with tension directly applied to the X-axis guide rods. Only 9 grams of filament used, printed in 30 minutes, yet it does the exact same job as tensioners which use much more material and take 3 times longer to print & assemble.
Original design here: http://www.thingiverse.com/thing:1669312.