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Leo,
Unless you want to make large numbers of these tubes, making your own is probably a case of diminishing return if not more expensive.
The mass produced ones have extra tube thickness on the end piece for the locking thread, a brass compression ring and internally threaded to act as baffling and reduce internal reflections and they are black anodised. I suspect the cost of the black anodising is probably almost as much as buying the extension tubes.
Have a roam around Ali Express, EBay, Amazon etc and see if you can find a finished piece that you can modify if you need something shorter.
The other option is to check scrap metal merchants or scrap bins of an engineering works. If you're lucky, they'll sell you stuff retrieved from the scrap bin at scrap weight cost or just give it to you.
Joe
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