Take a big array (40x40) and put a Frank-Read source a little off center to the right. Apply a large enough positive shear stress to activate the source and let it run until a full loop is formed. Click on the edge of each of the vacancies to turn it back to a normal atom. Then run briefly to anneal out any trash near the position of the (former) source. You can make the loop larger or smaller with appropriate choice of the stress.