I take 12 inch pieces of rebar and pound them into the ground 6 inches deep every three feet on each side of the row for the length of the row, or in your case, the length of the your garden. Then I take a length of pvc piping (a light enough gauge that it will bend.) Then I slide the tubing over one rebar and bend the other end over to slide over the rebar on the opposite side. Do this down the row, brace the whole thing by screwing a piece of wood along the top of the arc of your cover and enclose with a heavy gauge plastic and just like that you have a row cover. You could also use copper tubing for this project, but pvc is less expensive and also you can buy the pvc clips to hold your plastic to the frame.

You can also use 1/2 or 3/4" thin wall PVC pipe. Just cut to the length you want to create how much of an arch you want and wedge the ends of each piece in the sides of the bed between the soil and the side boards. Space then 2-3' apart depending on how much wind you get. They sell plastic pipe clips of the same diameters you can then use to clip the plastic to the pipes pieces.

Dave

Here is a link that might be useful: MEN - Low grow tunnels