Our steel tarps are made with 22oz vinyl with two runs of seatbelt webbing down the length of the tarp per side. The first run is 30 inches from the edge of the tarp and the second run is 60 inches from the edge. D-rings are spaced every two feet.  Grommets are along the edge of the tarp spaced every two feet as well.(Actual tarp measures 21'10" by 24' with an 8'x8' flap) See drawing.