One thing I am not certain of/familiar with is the securing of the PVC to the wall. I know that when using an expansion coupling the PVC needs to be somewhat loosely mounted to the exterior wall so that the expansion joint can move freely.
You are correct that if you rigidly fasten PVC immediately below the expansion joint, you defeat the purpose of even having an expansion joint. That's a common mistake you see in practice, from an installer who doesn't see the issue. You'd either need a strap that allows for axial movement, or you'd need to strap above the expansion joint. If you can find a strap that fits it, you can in concept strap the upper body of the expansion joint as Electrofelon has demonstrated. There isn't a manufacturing standard for these fittings to coincide with the next size up in their outer diameter, so it isn't a guarantee that a strap will fit it.
Generally speaking, plastic PVC straps on PVC will allow axial movement, while metal straps will not. The plastic straps grip it more loosely with a lower friction coefficient, and secure it primarily against lateral movement. It is OK to use metal straps on PVC, as long as it is a section of PVC where you don't need to accommodate axial movement and thermal expansion.
