re: Pergola Cover Options

Cross slats in opposite direction on the top of the pergola is the correct answer here.
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If you don't like that idea, you could always get a canvas (sheer or solid) with 4 Rings sewn onto the corners (Potentially one in the Middle). Put 4 J-hooks onto the corner posts (Potentially one at the top in the center). And spread it out to give you shade. Should take less than a minute to put up or take down whenever you want. And your wife can have decorative ones on the other 3 posts to make the look symmetrical when the under-cover one isn't in use.