For an adjustable tensioner you need to add a 624 bearing, a 4mm screw, and a 3mm screw to this part. It's designed for mounting on the top rod on the right end of the X gantry. Of course your x axis needs to be set up so the bottom span of the belt attaches to the x carriage. This tensioner will probably work fine inverted on the bottom rod too.
The parametric SCAD file also allows you to print a non-adjustable part at your adjusted tension. A sample non-adjustable one is included in this Thing. You can also adjust other parameters, such as wall thickness, bearing screw size, etc.