as shown in fig1 shown in fig2 and 3 is the rectangular sheet 10 as edge finished the sheet 10 has been folded about each fold line 14 so that each edge 11 of the sheet 10 lies atop the sheet and inwardly of one of the fold lines 14 and portions of the top surface 16 of the sheet 10 face each other the facing portions of the top surface 16 of the sheet 10 have then been sewn together by means of a plurality of conventional stitches 18 to secure such surface portions to one another thereby forming a hem preferably the stitches 18 are located between each edge 11 and its adjacent holes 12 in the sheet 10 the resulting sewn portions of the sheet 10 form a tube 20 along each side of the sheet 10 as edge finished and each tube 20 constitutes a hollow conduit which extends continuously along substantially the entire length of one of the fold lines 14 of the sheet 10 for example tube 20a extends continuously along substantially the entire length of fold line 14a on the bottom side of sheet 10 as edge finished as also shown in fig2 and 3 the sheet 10 as edge finished has a tie down line 22 loosely contained within each.