Ed,
I'm not sure what you mean by a "large knife". If it's the size of the knives I usually carry, then I'd humbly offer the advice that you reconsider skeletonized handles.
As far as sheaths go, I make my own Concealex sheaths to fit a Blade Rigger SB3 rig. I'd be happy to make one for you as well. If you'd like to know what my sheaths are like, I'd be happy to offer references (I make other sheath types as well).
Now, I carried this knife almost every day while we were on vacation last week:
It has a 12.5 inch blade, 19 inches overall, and concealed easily under a light coat.
These I carry from time to time as well.
11" blades, 17.5 OAL. I can conceal these under the overshirts my wife makes for me.
All the knives above are by Jerry Hossom, who makes the best fighters I've ever handled. His turn around time is also quick, compared to other makers.
Just a thought . . .
Matt
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Waxes Eloquent, Leader of the Terrible Ironic
Horde and Sarcastic Brain Spewer