Struggling with a block for an AC Attenuator/Silencer. thanks

[AMD-JEC]

I am trying to create a dynamic block that is scalable (yellow circle), but i want to keep the 50mm width on the top and bottom rectangles the same (circled in red)

im pretty new to dynamic blocks as never used them before in the 15 years i have used cad

many thanks for the help

[Libbya]

Please give a more precise description of what you would like to have happen.  Drawing the linework at different sizes would be a good way to relay that information.  I'm specifically curious about how you would like the red spline to be scaled.  Here's a screencast that shows how to do my best guess.

[AMD-JEC]

apologies for the delay in replying. that looks great thank you. the red spline is meant to represent the airflow inside the attenuator so would need to stay scaled within the inner rectangle. thank you