I bought a cheap printer off ebay, CTC clone, Geeetech, 5 mm plywood. The X axis idler/tensioner self destructed - see photos.
The difficulty with this part is inserting the combination of bearings, pulley and shaft into a single piece of plastic without moving the pulley on the shaft. On my example the pulley won't move and I am reluctant to use much force.
Following design  I initially split the part horizontally into two pieces. The split line is slightly above the centre of the bearings and as a result they snap into the bottom part.
For the second version (files suffixed 2) I used a couple of small self tapping screws (1.7 mm thread x 7 mm thread length) to hold the two pieces together.
For the fifth version (files suffixed 5) I split the part vertically into three pieces and used the tensioning bolt and an extra M3 nut to hold it together. The middle piece has a slot allowing it to move outwards for the shaft to be inserted and the back piece has a peg to lock the middle piece in place when the nut is tightened. This version prints well and is the best solution.
Eventually I learnt that idler pulleys that run off centre can affect print quality and replaced the pulley that came with the machine with a metal one. Version six of the tensioner is for this type of pulley, no messing about with multiple parts, a simple axle through the bearing which is part of the pulley.
OpenSCAD source included.