Forget barge...Forget Contact cement.....get some Leather Weld ( Jantz, K&G,TKS,etc.).
It won't even need stitching (But I still do it anyway...most of the time). I use heavy black dyed leather for a lot of tool and blade covers. Even with the slick polished side, the leather weld bonds tight and fast.
Here is what I do.
Cut the top ,bottom, and welt. Apply Leather Weld to the inner mark on the bottom piece. Set the welt in place, adjust it as needed, pressing it down to get a tack. Once in place and starting to "tack" (about 1-2min.) set a board on the piece,wait about 30 seconds, lift and check, set it back on and place a weight if you wish on the board. Wait five minutes. Apply Leather Weld to the welt top, put on the top piece, do as before, once all is right, put on the board and let set for about 30 minutes. The sheath will be dead flat and super strong. You can work on it now, but I let it set overnight. I have never had a joint fail( You have to cut the leather apart to separate it) with this product. For odd shapes or built up welts, use clamps and fingers. It sets fast.
Stacy