1. Use Default instead Evenly.
2. Make Compose segment out of the mullion+empty segment (in RC4 you can even leave transform node empty).
3. Set random size for the Empty Segment/Transform Node with Random parameter. Set min/max size with exported parameters. The important part is to set Random Mode to Segment so each segment will have a different size.
4. For Corners/Start/End use similar Compose method, just pay attention to the order: Empty+Mullion, Mulion+Empty, Empty+mullion+empty...etc and Empty/Transform Segment have Fixed side controlled by an Exported parameter.
That is it.