This x-belt tensioner works with the original right side bearing block that came with the printer, so you do not need to print a new one. It also tightens against the x-axis rods instead of the right bearing block. This is to prevent binding along the z-axis when the x-belt is tightened.

Parts needed:

The x-belt will need to be approx 4in (10cm) longer.
Machine Screw 4mm x 30mm with hex nut, two of each.
Machine screw, 3mm x 24mm with lock nut (the kind with a plastic insert)
Remove and reuse the existing x-belt pulley.

Installation:

After printing the tensioner, sand the inside of the rectangular slot, if necessary, until the x-belt pully fits inside loosely. Make sure the 4mm screws fit into the two holes on either side of the slot. Enlarge them with a drill bit if needed. Embed the hex nuts into the block at the hex shaped recesses of the 4mm screw holes: Thread a nut onto the 4mm screw, then hold it with a pair of pliers and heat the nut for 5 to 10  seconds with a small torch or cigarette lighter. Carefully press the nut straight down into the hex recess, then repeat for the other one. Install the pulley into the slot and secure it with the 3mm screw & lock nut. Do not tighten it too much, the pulley needs to be able to spin freely. Mount the completed tensioner, with a new belt, as shown in the photo. Tighten the 4mm screws evenly until you remove all slack from the belt.