An in-bar end mirror like the Cateye models will work with a full width twistgrip, since the latter doesn't have to press against the mirror stub. Just cut off the end cap of the twistgrip rubber and tighten on the twistgrip with a fractional gap to the mirror insert stub.
A split twistgrip is another solution as HarryB said.
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A split twistgrip is another solution as HarryB said.
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