UPDATE April 11, 2018
When I was sharing the thing here, I had installed that "S" size in my AnyCubic I3 Mega on X and Y belts. Recently, I have observed that the degree of bending increased due to the tension and PLA's behaviour under stress. Besides, I start to see some vibration marks on the sides of printed parts. Thus I decided to make it a bit more beefy. I hope, this way, it deforms much less then the present one. You can also find that the flange of the middle finger has been removed for easy installation.
Updated file has "S" size and if needed, more than one can be used on the same belt.
I use to have a M shaped thick metal wire for tightening up my X axis belt. Last week, I designed the one I indicated as (L)arge here and printed by using PLA. When I tried to place it on the belt, I realized that this is too thick for the job. Then I decided to have different sizes. So here we are, three different sizes from small to large one. Medium one fits on my X axis belt, whereas the small one is perfect for the Y axis. However, I must mention that the tension bends this thin parts a little bit, but the good news is that they do not flex under (my) peak loads. So far, I did not see any ghosting on printed parts surfaces. To be on the safe side, It might be a good idea to have a solid infill for these parts to add up their rigidity a bit.