AutoCAD Inventor :: Centering Parts To Assembly Reference

I am trying to find a simple way to center part(s) to some reference within an assembly such that if any part sizes are changed, then the assembly will stay symmetrical when updated. I would imagine that this is simply possible without adding any constraint containing a numeric value.

See attached part and assembly files showing a 4 piece frame with the angles unconstrained NE to SW. All i want to do is maintain symmetry of the frame if any parts sizes (cross-sectional or length) are changed. I've tried to add a centered plane to the angle and aligning that with a sketch point made in the assembly but i can't select any sketch features when i try to apply an assembly constraint.