Anet A8 X-Axis Retainer Clip

This retainer clips over the right-hand X-axis carriage and registers inside it and against the X-axis guide rods preventing them from pushing out. This will allow you to tighten the X-axis belt as tight as you want without misaligning/bending the Z-axis guide rods or screws.

This also prevents you from needing to buy a longer X-axis belt as most tensioners that push against the X-axis rods require.