I disagree with Clay's comments. True, Mike might have said that, but if I recall correctly he also said that he didn't test the Zytel version on his neck sheath. After all, he developed them initially for the G-10 handle BF Native.
I own both a BF Native and a Zytel Native. I have tested and carried both in Mike's neck sheath (I own two Concealex Native sheaths). In fact, just to be sure, I have just tested the Zytel version on his neck sheath once more. The knife is completely secure, with no loosening. In fact, I prefer to carry the Zytel version in sheath instead, since it is much more comfortable due to its much lighter weight. I have had no problems or doubts whatsoever about the effectivenes of this sheath when hilding either the Zytel or G-10 version.
Bteel, too bad you haven't had luck contacting Mike. That hasn't been my experience. In fact, I never purchased directly from him, but I did communicate with him a few months ago and he offered to send me a catalog if I emailed him my address. I did so, and forgot about the matter. Sure enough, not much later, I got his catalog in the mail. He also has hung around the Spyderco forum in BF very often, and has always had many useful comments about his sheaths and materials. I'm thinking about ordering a neck sheath from him for my Stamate in the future.
Good luck finding a neck sheath for the Native! It is well worth it for what I believe to be Spyderco's finest knife (along with the Starmate).
-Flood