String with balloons attached may also make a string curve upward though. A solid metal that doesn't bend easy would be fine imo.
But a glober might not believe that the metal bars are straight. A tight string has to be very close to straight, and if it curves, its a simple matter to know which way it's curving.
Without the balloons, a some of your fellow flat earthers might say that it was just sagging and the apparent upward bulge of the horizon was just by comparison to the sagging string, which is why the balloons are important - *if* there was any measurable sag, it would be up, not down, so it makes the experiment rather bulletproof.
However we could always try the straight metal edges, and if that showed a curve then some are going doubt it, and then we can try with a tight string, and if that still shows a curve then we can try the tight string with the balloons and well then the horizon can't be curved.
One advantage of the string is that if it will be a lot easier to set up a perfectly straight 10 ft string up in the mountains as compared to trying to hike up a 10 ft metal bar and keeping it straight. And the reason it's important for it to be long is so you can have it go all the way across your field of view while still being some distance from the camera so that it can be in focus as well as the horizon being in focus. If the bar is too close to the camera, either it or the horizon will be blurry and it'll blur the issue.
Another advantage of the string is that it won't block the view above or blow itself like the metal bar will.
As you can tell, I'm very excited to get up in the mountains with some string and balloons, but at the moment we got record snows here and I need chains just to get to the grocery store so there's no mountain trips for me for a while.