If you make soap in big blocks, you may find cutting with a knife, dough scraper, or cheese wire frustrating. You can make very inexpensive and excellent soap cutters using two wooden mitre boxes, two guitar strings, four screws, and two cup hooks. You may find these are the only cutters you ever need.
Screw one screw into one side of the mitre box, about one-third of the way along the length. Screw another one on the other side at the same place. Screw in the cup hook about ½ inch away from the screw. Screw the cup-hook all the way in, then back it out so that just a few threads are in the wood.
Cut a length of medium-gauge guitar string long enough to wrap around one screw. Go over the sides of the mitre box; go around the other screw and wrap, onto the cup hook. Leave a little extra on the ends. Make a loop at one end, twisting the wire to make sure it is fast. Loop that around the solo screw. Tightly pass the wire over the top of the mitre box, around the other screw, and around the cup hook.
Holding the wire tight, twist the cup hook so it starts to go into the wood, taking up and tightening the wire. Since there aren't a lot of turns on the screw of the cup hook, be sure the wire is quite tight before you start to turn it. Twist until you get a somewhat musical note from the wire.
Repeat with the other mitre box, except this time pass the wire through one of the cuts for holding the saw. Run your blocks of soap through the “big” cut first, then through the smaller cut to make bars. Clean off the cutters between uses.
Mitre box log cutter