Customized Xamarin.Forms Frame that allows you to specify individual corners to round.

Android_CustomFrameRenderer.cs

using Android.Content;
using Android.Graphics.Drawables;
using MyApp.Controls;
using MyApp.Droid.Renderers;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(CustomFrame), typeof(CustomFrameRenderer))]
namespace MyApp.Droid.Renderers
{
    public class CustomFrameRenderer : FrameRenderer
    {
        public CustomFrameRenderer(Context context)
            : base(context)
        {
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
        {
            base.OnElementChanged(e);
            if (e.NewElement != null && e.OldElement == null)
            {
                GradientDrawable drawable = new GradientDrawable();

                var frame = e.NewElement as CustomFrame;
                float[] radius = new float[8];

                radius[0] = frame.RoundTopLeft ? Context.ToPixels(frame.CornerRadius) : 0;     //Top Left corner
                radius[1] = frame.RoundTopLeft ? Context.ToPixels(frame.CornerRadius) : 0;     //Top Left corner
                radius[2] = frame.RoundTopRight ? Context.ToPixels(frame.CornerRadius) : 0;    //Top Right corner
                radius[3] = frame.RoundTopRight ? Context.ToPixels(frame.CornerRadius) : 0;    //Top Right corner
                radius[4] = frame.RoundBottomRight ? Context.ToPixels(frame.CornerRadius) : 0; //Bottom Right corner
                radius[5] = frame.RoundBottomRight ? Context.ToPixels(frame.CornerRadius) : 0; //Bottom Right corner
                radius[6] = frame.RoundBottomLeft ? Context.ToPixels(frame.CornerRadius) : 0;  //Bottom Left corner
                radius[7] = frame.RoundBottomLeft ? Context.ToPixels(frame.CornerRadius) : 0;  //Bottom Left corner
                drawable.SetCornerRadii(radius);
                drawable.SetColor(frame.BackgroundColor.ToAndroid());
                this.SetBackground(drawable);
            }
        }
    }
}

CustomFrame.cs

using Xamarin.Forms;
namespace MyApp.Controls
{
    public class CustomFrame : Frame
    {
        public static readonly BindableProperty RoundTopLeftProperty = BindableProperty.Create(nameof(RoundTopLeft), typeof(bool), typeof(CustomFrame), false);
        public static readonly BindableProperty RoundTopRightProperty = BindableProperty.Create(nameof(RoundTopRight), typeof(bool), typeof(CustomFrame), false);
        public static readonly BindableProperty RoundBottomLeftProperty = BindableProperty.Create(nameof(RoundBottomLeft), typeof(bool), typeof(CustomFrame), false);
        public static readonly BindableProperty RoundBottomRightProperty = BindableProperty.Create(nameof(RoundBottomRight), typeof(bool), typeof(CustomFrame), false);

        public bool RoundTopLeft
        {
            get { return (bool)GetValue(RoundTopLeftProperty); }
            set { SetValue(RoundTopLeftProperty, value); }
        }

        public bool RoundTopRight
        {
            get { return (bool)GetValue(RoundTopRightProperty); }
            set { SetValue(RoundTopLeftProperty, value); }
        }

        public bool RoundBottomLeft
        {
            get { return (bool)GetValue(RoundBottomLeftProperty); }
            set { SetValue(RoundTopLeftProperty, value); }
        }

        public bool RoundBottomRight
        {
            get { return (bool)GetValue(RoundBottomRightProperty); }
            set { SetValue(RoundTopLeftProperty, value); }
        }
    }
}

iOS_CustomFrameRenderer.cs

using CoreAnimation;
using MyApp.Controls;
using MyApp.iOS.Renderers;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(CustomFrame), typeof(CustomFrameRenderer))]
namespace MyApp.iOS.Renderers
{
    public class CustomFrameRenderer : FrameRenderer
    {
        public override void LayoutSublayersOfLayer(CALayer layer)
        {
            base.LayoutSublayersOfLayer(layer);

            var frame = Element as CustomFrame;

            CACornerMask corners = new CACornerMask();
            if (frame.RoundTopLeft)
                corners = CACornerMask.MinXMinYCorner;
            if (frame.RoundTopRight)
                corners = corners | CACornerMask.MaxXMinYCorner;
            if (frame.RoundBottomLeft)
                corners = corners | CACornerMask.MaxXMaxYCorner;
            if (frame.RoundBottomRight)
                corners = corners | CACornerMask.MinXMaxYCorner;

            this.ClipsToBounds = true;
            this.Layer.MaskedCorners = corners;
        }
    }
}