I use wax seals and they should be made out of metal or something that is heat resistant. I also work with rubber stamps and they will not hold up to the repeated exposure to the heat. The wax is not really wax, but a stiffer material that melts and has a wick down the center. the wick is lit with a match and allowed to melt and dripped onto the back of the envelope. It is very hot, enough to cause a good blister. I know from experience. I would buy a brass rod, engrave it with a dremel or foredom and then turn a handle for it from wood. I am not sure that the corian wouldn't be damaged over time by the heat. Shouldn't be hard to make, just remember that the letter should be a reverse so that when it is used the letter is indented in the wax.