The key factors for a good seal at the butt seem to be dimensionally stable wood, (preferably quarter-sawn), a close fit to the horn, a good seal around the pins, and (surprise!) no holes going through the plug. Pin your strap attachment to the wood, not through it. A horn with a strap hanger pinned through the wood, or with a threaded or plugged "filler", is much more likely to leak.
I gave up on wax a while back, after the wax sealing a horn melted and contaminated my powder. My most-carried horn was made in camp, and sealed with pine pitch (or tar, I don't recall). About 15 years later, it spent a few days under water after a canoe wreck. When recovered, the powder was fine but the wood plug had taken on enough moisture that when it dried out it cracked. I eventually made a new plug, sealed it with pine tar, and expect it to last for at least another 20 years.