Now mark a North-South line on the top of the carved donut. Magnetic North-South is fine. Dig a large pit and obtain a large quantity of damp sand. Bury the foam form upside-down in the pit with the bottom surface even with the top of the sand. Pack the sand well as you fill the pit.
Fine sand produces a smooth surface but coarse sand gives a lovely rough-cut look.
Pour in sufficient molten material to fill the form. The foam will evaporate and the melt will flow in and fill in the space created. Aluminum is lightweight and durable but for a classic pre-Gaelic look try molten granite. Dig out the stonehenge and place it right-side up. Clean and use the North-South line to align.