I have some grip tape on its way to me arriving tomorrow.
Thinking 3 or 4 rotations strategically placed on the 21700 body ought to do the trick.
Gene himself recommended me use grip tape to stop any slip in the hand.
It's a very cheap and easy fix, if the tape wears out over time, easy to cut it off and reapply some more.
I love the HD 18650 with the 21700 body, it's a rockstar... hard to
beat the performance and keep the same size/form factor. Especially having that Hi/Lo ring. It's very nice.
It's interesting Gene decided on this type of finish for his lights, I'd imagine one wouldn't ordinarily want such a slick/slippery finish on a flashlight. But If some grip tape will correct the issue, I suppose it really isn't a huge deal.
I shall report back later with the results of my endeavor!