I've been using construction twine, which is synthetic and has a twist. But I imagine just about any burnable string would work. For example, kit string is quite strong. As long as it burns, and doesn't readily break when you pull on it, it will work.
Re: overall strength, keep in mind that the total force at any given time is still pretty low. For example, if all three C6's and the center D12 peaked at the same instant, the peak thrust would be ~72N, about 16 pounds. While I'm no Charles Atlas, I can readily impart 16 pounds of force to test a string
So if you can't break it by stoutly pulling on it, it's surely strong enough - no fancy test equipment required
Doug
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