Try using only one strap fastened across the width not length and only tighten lightly. If you use it nearer the topmost part of the roof you should still get the run off.
The straps you are using are great but very very strong and will crack the sheet if not carefully tightened.
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Buckled Straps 2 Pack 2.5m x 25mm Cam Buckle Tie Down Lashing.
They are £2.35 for two including postage. They are very easy to use and you should not over tighten.
You could use a 2" board at the top and 1" in the middle for run off.
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