Try this
var ContainerNode = MindFusion.Diagramming.ContainerNode;
var originalDraw = ContainerNode.prototype.updateCanvasElements;
ContainerNode.prototype.updateCanvasElements = function ()
{
originalDraw.apply(this);
var w = this.bounds.width;
var h = this.bounds.height;
var r = 3;
var pathString = String.format(
'M0,{2} L0,{4} Q0,{1},{2},{1} L{3},{1} Q{0},{1},{0},{4} L{0},{2} Q{0},0,{3},0 L{2},0 Q0,0,0,{2} Z',
w, h, r, w - r, h - r);
var path = new MindFusion.Drawing.Path(pathString);
path.brush = this.brush;
path.transform.translate(this.bounds.x, this.bounds.y);
this.graphicsContainer.content.splice(0, 1, path);
};
I hope that helps,
Stoyan