[Chris]

It's pretty easy to make your own. Copy the text below exactly and save it to an ordinary text file called OpenBox.graffleshapes. Then put this in the folder Library:Application Support:OmniGraffle:Shapes under your home folder. (You may have to create it. Then open OG and you'll see your shape next to the regular rectangle.

{
    shapes = (
	{
	    InspectorGroup = 1.5;
	    ShapeName = "OpenBox";
	    FillPath = {
		elements = (
		    {element = MOVETO; point = "{-0.5, -0.5}"; },
		    {element = LINETO; point = "{0.5, -0.5}"; },
		    {element = LINETO; point = "{0.5, 0.5}"; },
		    {element = LINETO; point = "{-0.5, 0.5}"; },
		    {element = CLOSE; }
		);
	    };
	    StrokePath = {
		elements = (
		    {element = MOVETO; point = "{-0.5, -0.5}"; },
		    {element = LINETO; point = "{-0.5, 0.5}"; },
		    {element = LINETO; point = "{0.5, 0.5}"; },
		    {element = LINETO; point = "{0.5, -0.5}"; }
		);
	    };
	}
    );
}