I know you said you are bolting it to the shed, but another eefective way of keeping it really held down is tie some lengths of string to the top of the frame and tie the other ends to the sandbags, that way it keeps the frame a bit more rigid too. I had a small one made out the same stuff as yours and I kept repairing it and repairing it for 3 years in the harsh wind (I live at the top of a hill, it's always windy) I gave up and built a wooden framed lean to with rigid plastic roofing sheets. Just keep it all tight al round with as little room to move as possible. Peace