I've handled Nathan's sgian and the antler is massive. Very nice, sharp Sheffield steel too!

Just wet moulding his current sheath is unlikely to counteract the problem completely. I'd suggest getting a slightly bigger sheath and wet moulded to the knife, so that part of the handle is covered by the leather. That way, when the sgian is pushed further into the hose to counter balance the weight of the handle, some of the sheath could still be under the garter ties.