Patrice, nice looking sheath, from the way the handle looks
it would appear that the handle gets smaller as it goes down to the blade?
No guard to help with retention it seems?
On a sheath like this, I think you are right, wet molding isn't a good retention method
to save this, if it were me, I would glue in a small thin layer of soft deer skin
or elk skin as if you were lining the inside.
What this does is then make it a wedge type fit, the knife will snuggle up against
the springy elk skin and help with retention
See this thread, I had a similar situation;
Shown here
BUT, as I don't make a lot of fixed blade sheaths any longer
some of the other folks might chime in with some other ideas
but in the short run, this might be one thing you could try.
Dry fit it in before gluing of course, you want it snug.
Best of luck!
G2