Upside down, bad. If you really, really, really must attach it to your shoulder strap, I'd go with one of the more secure knife/single pistol mag pouches that is designed to be carried horizontally on the belt, such as the single cell SpecOps SuperSheath, and THEN add a bit of bicycle inner tube around the outside of the sheath to keep it TIGHT.
Either that, or a small utility pouch attached to your shoulder strap that would hold your Wave, a flashlight, lip balm, compass, you know, the stuff you need frequently so you don't want to put it in your pack proper.
One question I suppose is what kind of pack are you using? Some of the newer styles ahve molle webbing on the shoulder strap, so you could add something VERY secure that has, say, velcro and a fastex buckle, such as the USGI or EssTac single pistol mag pouches. But security is going to be a big concern no matter what you have up there.
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-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.