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I was thinking to make the pouch wider and deeper than the knife by a mm or 2 and to make the holster bit shallower on the sides or sides and front than the knife, the flap will stop it escaping.

Sounds like you have your next project then. Just make sure it is easy to get in and out..

Oh come on this is John you are speaking to, it would obviously be an old Avon aftershave bottle :)