I'm trying to find the link for you but I can't. There is a cutter that you could probably make since you are handy, it looks doable.
It is a rectangular frame basically, with the wires strung across it. You would prop it up on the two short sides, and then sit the soap on top of it (top down), placing a board on the bottom of the loaf, and then slowly push the soap log through the wires, cutting them.
I drew you a picture in paint... forgive my art plz.
and it might have those little tightening things on each side of the string or something.