Water cap that fits over the slider between the cap and cab?

I like to leave my slider open when parked to let air in the cab, especially in hot weather. The only problem is rain getting in between my cap and the cab. I am thinking of a water cap that fits over the slider between the cap and cab. I tried a piece of foam pipe insulation that you use to keep pipes from freezing. Worked good but the noise it makes rubbing on the cab is maddening not to mention the fact that it will probably scuff the paint. Any ideas?

 

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