I've tried to make a couple shoulder holsters. The main thing is to allow for plenty of adjustment for the customer, as he will need to adjust depending on wear and activity. I had one horizontal, and think that works best for autos, but my other was for a S&W 500, and due to its size, even with a snub barrel, had to to vertical. Wide straps across both shoulders, soft leather, and a good back center pivot to allow things to move.