Couple questions I would ask. What is the 'build' of the guy requesting the sheath, and is he concerned with quick access to the knife?
If possible, I'd go with a single, small, straight guard on the knife. That way you could go with a pouch style sheath that would cover at least half of the handle too. With the flare at the end from the stag cap, withdrawing the knife from the sheath shouldn't be a problem.
If the customer's waistline allows, one thing that works well to keep the knife high and out of the way is a wide belt loop stitched to the back of the sheath, below the handle. That way it rides like a pistol holster, out of the way of a seat or saddle. You can also cant the loop a bit to tilt the handle forward for easier access. For the really svelte types, a simple loop on the edge, with the sheath riding inside the belt holds it even tighter to the body and out of the way.
I have a friend that does his sheaths out of 12-14 oz. harness leather. When a knife slides into one of those sheaths it literally snaps into place, and it is so stiff that I can't imagine something like that needing riveting or a kydex liner.
To really be on the safe side, a thong hole drilled through the handle, and a corresponding leather thong loop on the sheath is a simple, but almost foolproof 'keeper' in case the horse does some antics. You push the loop through the thong hole, then drape the loop over the top of the knife. I had an old Puma White Hunter like that, and if the snap provided on the sheath failed, the loop would keep the knife blade from retracting more than a half inch or so from the sheath.
madpoet