One other thing you might do would be to layer in a piece of deer or elk skin
the kind that is soft and pliable, along the spine side of the sheath, in like
a half moon sort of thing, taper the leather down near the opening so it doesn't
catch on that, I'd have it go in as far as you can, to help fasten it with the barge
cement.
What this does is provide a layer of springy material that will wedge the knife in place
so it helps to keep a tighter hold on the handle for you.
I had to do this type of thing for a knife with no guard, no scales, just a straight piece of
damascus, here's a link to that thread to view, it was a challenge to come up with how to
do that without a flap type sheath but it worked very well and the knife is secure.
See this
Thread here
G2