After deciding to use this belt tensioner https://www.thingiverse.com/thing:2253143, I was searching also for a way to hold the x-axis GT2 16T pulley because these I have do not fit into the normal Anet A8 pulley holder. Nothing found and so I had to find another way. I found this X-axis belt tensioner here https://www.thingiverse.com/thing:2196516 and for my luck the designer has also uploaded a "CAD" (Blender) file for it.
Print it like it is in the file for best strength.
It is important to add lightweight, easy to remove supports, especially for the place where the pulley sits. Try the layers simulation in your slicer software to see what looks like it could be easy removed after printing.
I used slic3r and the supports I defined (see screenshot) were not so optimal for easy removing...
To slice the thing with slic3r I had to correct the blender file. There were a few Non-Manifold vertices / edges that I have removed and corrected. This is also the problem that some people mention on the original tensioner design. Cura seems not to have problems with this but I get better results with slic3r, so I had to make it working...
PS. This was my first contact ever with Blender... :-)