Family modeling - how to create this diagonal element?

[tfollisBJKUS]

I am trying to model one prong of a light fixture like this: https://www.lightformshop.com/suspension-lights/chandeliers-lighting/z-bar-suspension-bird-shape

I've modeled it pretty simply. One vertical solid extrusion cut by one horizontal void extrusion makes one full prong. It looks believable and functions fine when placed into a host family with 4 other prongs to make the full light fixture. But I can't seem to constrain it in such a way that I can control the height of the family without the void extrusion malfunctioning and misshaping itself. I've asked around my office and it's puzzling some of the guys here, though I imagine the solution is probably pretty simple. Does anyone know how to build this? Screenshots here explaining the problem.

here's how it's functioning when I move the ref planes up or down.

and here's how I want it to work

Thanks in advance!

   

   

[constantin.stroescu]

I'd use another method to create the Bird Shape form....I'd use a sweep.

I suggest to use a parametric Adaptive Generic Model as template ...but not quite...something like in my following screencast:

https://www.youtube.com/watch?v=mI8m9klLuOw

[Mike.FORM]

Add some additional reference planes and and lock the dimensions.

Then in the void sketch align and lock the end pints of the angled lines the the intersecting reference planes. You will need to TAB until the end points are selected for aligning.