Hey, Zach. Welcome to the forum. While it's certainly possible to make a mold of a commercially available insert and then modify that to get your own sound, I think you'll find that most guys frown on this approach as it's too close to copying someone else's work. I think you'd be better served by purchasing a public jig and developing your own toneboard from there. It's basically the same principle you describe except you start with a jig that's designed for this purpose.
So much of custom call making is based on the reputation of the call maker. And trust me, you don't want to be known as "that guy" who got started by copying someone else's work. I've seen it happen before and it's not pretty. The knowledge you gain by creating your own insert and finding out what does and doesn't work is priceless. There's a lot of respect from other call makers to be gained here as well. Lots of folks on THO are more than willing to help by offering advice on diagnosing problems and solutions.
Those are my two cents anyway. I'm sure others will speak up on this topic and give you guidance.
