Luckily, as the cross sectional area shrinks, the price of valves shrinks. As for pressure inside the horn theoretically bursting it, well, I think you'll be OK but if you insist, protection gear would be little more than hinges along one edge of a panel and a spring on the other, not much different than a very large turbocharger wastegate (very large as in stationary multimegawatt class diesel size)