You're quite right, that could be done!

However, I didn't want to wreck the original guard (they're massively more expensive than the XTR lower!) Also the original is quite bulky and you'd have to retain some of the front portion because you'd need the front mountings as it's not flexible enough to pull tight against the fork legs. The idea of using the XTR lower is that it small, light, neat, matt black and isn't really noticeable.

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