You could put the seam running down the long axis of the tarp, leaving you with two 18″ x 9′ leftover strips of DCF. That way, you get a strong flat felled seam running down the apex of the tarp. With the leftovers, you could sew 7 ft sections of those two strips together into a 34″ x 84″ wide DCF poncho, though it would be pretty thin if using 0.51 DCF. You’d have just enough material left for a 20′ x 10′ x 6′ backpack liner, or the poncho hood and a few stuff sacks.