Welt thickness and a close fit will make the sheath keep from being loose and slipping off too easily. Make the edge welt thinner than the spine welt. Do this by gluing it up as you have it in the photo, and after the glue (I use leatherweld) is dry, sand or shave down the welt at the edge to half the thickness of the spine. You can wet shape the sheath, too, if needed.
TIP:
Clamp the sheath parts between two boards while drying to assure the sheath doesn't "warp", and to assure a strong and invisible joint. Do the welt and back first, check the blade fit and trim if needed, then glue on the top side. Once the sheath is dry, sand the edges on the belt grinder to get a smooth and even edge. Sand before stitching! Use a sharp 120 grit belt to get it even and then a sharp 400 grit to get it smooth. Burnish with a hardwood slicker. Applying edge dressing after dying the sheath makes a very professional looking edge.
Normally, a properly fitted strap over the guard or around the handle retains the knife in the sheath. On bowies and other large blades that are not really going to be carried, I often make a half sheath like the one you have and call it a blade cover. A thong run through a hole in the spine corner of the sheath can be wrapped/tied around the handle/guard.
As you have been told, the throat has to be as wide as the widest part of the blade. Heavily curved blades need to have the clearance in the sheath worked out, too, or the blade will bind as it inserts/removes and cut the welt and stitching. Your bowie is fine as far as any curvature goes.