The original tensioner for the CTC i3 caught on a bolt in the frame as the X carriage moved up and down the Z rods because it was too wide, it also also rubbed on the frame.
Designed this which is a slimmer version designed to use an M3 25mm bolt with a metal GT2 20T idler pulley instead of the plastic 20T motor gear it originally used.
Use the bolt from the original tensioner to go through the centre of the part, this will then go through the X carriage mount on the Z rods.
Add an M3 washer to a 25mm M3 bolt and put it through the side so the head of the bolt is on the side which will face the frame. Once through the first side place the pully in the belt and slide the bolt through the GT2 20T idler. Then go through the other side of the part add another M3 washer and tighten 2 M3 nuts onto the bolt to avoid them coming loose.