public class Carousel : ItemsControl
{
    private ScrollViewer _scrollviewer;

    private ScrollViewer ScrollViewer => _scrollviewer ?? (_scrollviewer = (ScrollViewer)GetTemplateChild("PART_scroller"));

    protected override void PrepareContainerForItemOverride(DependencyObject element, object item)
    {
        if (element is FrameworkElement fe)
        {
            var sv = ScrollViewer;
            fe.SetBinding(MaxWidthProperty, new Binding { Path = new PropertyPath(nameof(ScrollViewer.ViewportWidth)), Source = sv, Converter = new ItemsSizeConverter() });
        }
        base.PrepareContainerForItemOverride(element, item);
    }

    class ItemsSizeConverter : IValueConverter
    {
        object IValueConverter.Convert(object value, Type targetType, object parameter, string language)
            => value is double d ? d * 0.9 : DependencyProperty.UnsetValue;
        object IValueConverter.ConvertBack(object value, Type targetType, object parameter, string language) => throw new NotImplementedException();
    }
}

public class CarouselPanel : StackPanel, IScrollSnapPointsInfo
{
    public const double OutsideGutter = 100;

    public CarouselPanel()
    {
        Orientation = Orientation.Horizontal;
        Padding = new Thickness(OutsideGutter, 0, OutsideGutter, 0);
    }

    IReadOnlyList<float> IScrollSnapPointsInfo.GetIrregularSnapPoints(Orientation orientation, SnapPointsAlignment alignment)
    {
        if (orientation == Orientation.Vertical)
            return Array.Empty<float>();

        return points().ToArray();

        IEnumerable<float> points()
        {
            yield return 0f;
            var x = OutsideGutter;// this.FindParent<ScrollViewer>().ViewportWidth * 0.1;
            foreach (var c in Children.OfType<FrameworkElement>())
            {
                yield return (float)(x + c.ActualWidth / 2);
                x += c.ActualWidth;
            }
            yield return (float)ActualWidth;
        }
    }
}