Diagram Cell Drop Target or Visual Cue?

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I have a working app with the diagram shown below. The diagram is intended to have a grid structure with rows and columns. I need to highlight the borders of a Cell area as a potential drop target when a Node is dragged from a Palette over the Cell area. How could I go about doing that?
                    <go:Diagram x:Name="FunctionBlockDiagram"  
                                AllowDrop="True" AllowScroll="True"
                                Model="{Binding DiagramModel}"  
                                LinkTemplate="{DynamicResource LinkTemplate}" 
                                NodeTemplate="{DynamicResource FunctionBlockTemplate}"  
                                SnapsToDevicePixels="True" >
                            <go:GridLayout Conditions="None" 
                                           CellSize="250, 300"
                                           Spacing="50 50"
                                           WrappingWidth="2400" />
                            <graph:FunctionBlockDraggingTool DragOverSnapArea="Diagram"  DropOntoEnabled="False" />
                            <graph:FunctionBlockLinkingTool />
                            <graph:FunctionBlockReLinkingTool />
                            <go:GridPattern CellSize="25, 25" >
                                <Path  go:GridPattern.Figure="HorizontalLine" >
                                        <SolidColorBrush Color="{DynamicResource LightGrayTop}"/>
                                <Path go:GridPattern.Figure="VerticalLine" >
                                        <SolidColorBrush Color="{DynamicResource LightGrayTop}"/>

So you want something different than what the Planogram sample does?
It would be different because you only want to highlight that one cell?

I would extend the DraggingTool (which you have already subclassed, I see) to put a temporary Rectangle in the appropriate place. It would be temporary because it would be in the “Tool” layer and not be data-bound.