How to create a Fixed Scrollable portion in CusotmScrollView in flutter?

Question

I am using a CustomScrollView and Slivers inside it and redendering the list using SliverChildBuilderDelegate. This works fine, but my requirement is to have a Fixed Container of height = height of screen/2 and inside that I want to render a list and and it can scroll as well, I am using CupertinoSliverNavigationBar [this cannot be replaced]. I am able to draw a Container and inside that I am able to show a list using Listview.builder, but the problem is I am unable to scroll the list, whenever I scroll it, it goes to top again, I want something Like, my CustomScrollView should be kept as it is, just I want to create a Scrollable Container of fixed height and the list inside that should scroll properly and CustomScrollView can also scroll, I have tried many widgets like Wrap, Rows, Columns, SingleChildScrollView, also tried NeverScrollPhysics() Behaviour for CustomScrollView but nothing is working out. Please Help!