>> my concern is that rain water would get trapped under it and make matters worse.
True! There would need to be air space under the plastic, and even so, I think you're right that it would make things worse.
>> and may again in the future, so if you have ideas, I'd love to hear them!
Only one thing based on my own shakiness on ladders lately. Think twice before you plan to climb up and down ladders repeatedly with your hands full of melons and fruit!
