25whaler

I am new to the board and excited about it. Its exactly what I was looking for. I was hoping I could call on some expertise with making lead lure inserts. I mostly make lures for offshore saltwater fishing. I need to make a lead mold that is square and can accept 3/16 brass tubing. See picture. I then put this tubing into a resin mold. After it cures I turn it on a lathe If anyone can assist in what type of material to make my mold out of for the lead I would greatly appreciate it. Also where to find the material. My experiment with high temp rtv was a disaster. The insert I was able to make ws made with angle iran and very difficult to pour. Thanks in advance for some advice