Using the Rectangle tool (I key), click and drag to create a rectangle.
Use Shift to create a square and Alt to start drawing from the center.
Use the Ctrl key while drawing to make underlying filled areas of existing contours unfilled:
The created contour will be added to the current element. To start drawing in the new element, press Esc or use the Glyph > New Element command before drawing.
You can create more rectangles using the Add button in the context Property bar where precise parameters are entered. X and Y are the coordinates of the selected point (the lower left corner by default). The Width and Height dropdowns in the Property bar give you quick access to the important font dimensions (caps height, x-height, main stem) so you can create rectangles even faster.