A quick remix of Murin's M42 macro extension tube. I chamfered the inner ring to make this print without supports, and adapted the size to standard extension tube ratios. It comes in a 10mm, 20mm, and 36mm size.
I would recommend first printing the 10mm to check the tolerances of your printer. If its not an exact fit as is, you can increase or decrease the x and y sizes in your slicer, while leaving the z the same. This allows you to easily make minor adjustments to the diameter.
As mentioned in the original, print this at 0.1mm or else the threads will be too thick to function properly.
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