Welding the nut to a floating piece of U channel is not a good idea. If the clamping surface is uneven you increase the likeliness of the nut placing an uneven load on the fastener with the U channel acting as a lever; I'm certain this is the reason for the original method of construction.
I would go at it from the side myself. It depends on how large of an aperture you need and what you're using to cut with and weld with though.
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