Just a thought: if wind is the main stress, rolling up the sides might expose them abruptly.
Is there something you can stick around the bed to slow the wind down? Brush? Slats?
Or maybe roll up just one side at first, or the ends.
I'm just guessing - I don't have experience under plastic YET.
I'm planning to start seeds indoors in trays, then move them outside when it's still too cold, but under a plastic hoop tunnel serving as a solar cold frame. Probably have to pull that away in the daytime after a while, and only put it back in late afternoon to keep them warm at night.
