Yes, I agree, sanding seems not possible due to the thin layer inside. However I still would like to have a 21700 tube, since this size format is adopted by more and more cell manufacturers. Now there are more and more decent 5000mAh 21700 choices (LG M50, Samsung 50E, Sony 21700VC7) and that cheap but still good 4000mAh LISHEN 21700; but when it comes to a high capacity 20700, NCR20700B seems the only choice.

1mm difference in diameter is almost negligible, so 21700 is more favorable to me, after all.