A loop using a separate piece of leather can be attached to the back easily, before sewing the sheath together. The problem I have when doing that, is that the loop gets squished like a frog under a semi when I press the sheath. They still work fine, but look terrible. Now you got me thinking, and that's dangerous. Okay, the upper portion of the loop could be sewn. The lower portion that is pressed flat could be riveted with a copper rivet. Carefully drill the rivet holes, trim the rivet as far as you dare. Round off the ragged tip that you trimmed, force the rivet into the sheath using long needle nosed pliers and work it through the hole. Slide a piece of 1/8" steel in under the rivet, to be an anvil, then rivet it down on a vise anvil, or similar hard surface. Protect the leather with a piece of poster board under it. I have done this with other sheaths, but have never tried it on one of the center seams. As long as you can work the rivet through the hole, it will do the job, and not look so bad. Thinking more on it. You should be able to do the same if you just left a tab to fold over like you said, after the sheath is pressed.