Email sent, alf.
ZDP - you've got the right idea. One way I did it was to tape a piece of cardboard (to the edge) in the "void" - shown in your first illustration - between the khukuri and your outline. Then form the sheath around the two of them.
That said, you can also make a quickdraw style by cutting away the spine area down to the "bend" in the khukuri. Works nicely.
Or, just form it like you would a regular sheath, but put the rivets outside of your outlined area. Then the khukuri never contacts the rivets and can be drawn with no problem. Use the rivets and a heat gun (or hair dryer) to adjust the draw.