With the smaller airplanes, we used to avoid using manufactured horns in the first place. They are just too heavy for the application.

We'd simply use some thin plywood. Use it like I've used the plastic horn remnant shown in the Skybolt photo. It's plenty strong enough and we'd strengthen the hole with epoxy, and it'd last forever too. And it cost nothing.

Cost almost nothing.
Weighs even less.
Easier to do.

Good modelers can make or fix anything.