Bench Support?

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Just got my mill on the bench and although it is strong enough to hold the mill, I noticed that the center of the bench is just slightly bowed which of course makes the bench not level. Here's a couple pics showing the bench and some channel stock I have which would fit perfectly inside the bench across the center of the bench keeping the top of the bench from bowing, or.... would it be better to put a piece across the middle of the bench front to back?