Hrmm... for a later period, I think this general pattern is great, and would match the lines of your knife beautifully with the right incised decoration:
http://www.lukasmaestlegoer.com/knives- ... rhauswehr/
BUT that long handle of yours is I think going to move the balance far enough back it will want to tip and fall. Plus you've mentioned a liking for the early medieval period, so you might consider a Seax style sheath:
http://myarmoury.com/talk/viewtopic.29767.html
... I think they're a little out of place for the Dunedain of Middle-earth, but that's just my eye. The Professor might well disagree: the more of real Saxon culture I study, the more of it I see in Eriador, so... maybe?
Finally, Greg's seax scabbard is I think a wonderful model, and carries the a similar fashion to what you describe. (You probably want to copy his strap arrangement for stability as well)
aaannd... shoot. I can't find that thread at the moment. I'm sure searching for his posts will turn up pictures.
.. those are my two pennies worth.
edit - one more thing. I've seen a few people do those seax-style sheaths with the brass parts replaced with bone, wood, or iron. That both reduces light reflection, and can be an awesome look in its own right. Anyhow, food for pondering.
Persona: Aerlinneth, Dúnedain of Amon Lendel c. TA 3010.