I had straight cuts on mine w/ rain caps and after bout 3 months the dinging of the caps started drivin me nuts. So now i'm runnin mitres and it's rained all week and so far i've havent had a problem.
You have way more tolerance than I do. I had a rain cap on my straight cut for about a week. Now it's sitting on a shelf. Sumbitch, those things are annoying! Now if its raing hard, I go out and put a lid from a can of Gator-aid mix on it. -
drill a small hole in the bottom of ur 90 so the water has some where to go....mine has been in snow and rain storms without being covered, and its a mittered cut....so no worries....... -
So now i'm runnin mitres and it's rained all week 
and so far i've havent had a problem.
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