aeroflash, I measured 36.5mm inner diameter (+-1mm, as I used a ruler) from the one I received from Ledean. As others have said the quality is excellent.
Edit: Diameter may be 36.5 +-1, but clearance through the threads is less, say 35.5 +-1, which was the point of the initial question I'm sure. I would say 2 18650s side by side wouldn't fit (I don't have two myself to actually test though).