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Bintel has a set of M48 extension tubes with a 5mm in it and I know Sidereal Trading brought in some thin stainless ones when I bought a few. You would have to stack a couple of the spacer washer ones to make up the 2.5 (Not sure you would get 2.56 exactly) and a threaded 5mm tube and spacer washers to get to 7.9mm. Other than that you would have to get something made I reckon.
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