Attach a hook, eye, or even a large screw to the four corner at the bottom of the frame. Get a roll of light aluminum wire from Lowe's/Home Depot. Tie one end to the first eye/hook/screw and just stretch the wire over your covering from corner to corner. This is exactly what I do but I stretch the wire about every 3', parallel to each other so that the wind can't blow under my solar blanket material and lift it.
This cheap stuff didn't last the year out. You can see the wiring if you look close.
Three years and this solar blanket material looks good as new.