Code

container.onUpdateVisuals = onUpdateVisuals;

function onUpdateVisuals(node)
{
    var shape = MindFusion.Diagramming.Shape.fromId('RoundRect');
    var path = new MindFusion.Drawing.Path(shape.params.outline);

    var shapeBounds = shape.defaultBounds;
    var bounds = node.getBounds();
    var x = bounds.width / shapeBounds.width;
    var y = bounds.height / shapeBounds.height;

    var matrix = new MindFusion.Drawing.Matrix();
    matrix.translate(bounds.x - bounds.width / 2, bounds.y - bounds.height / 2);
    matrix.scale(x, y);
    matrix.translate(
        bounds.width / x - shapeBounds.x - shapeBounds.width / 2,
        bounds.height / y - shapeBounds.y - shapeBounds.height / 2);

    path.transform = matrix;
    node.graphicsContainer.content.splice(0, 1, path);
} 

Code

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);
};