Hi,
It is probably too late to help you with your soap cutting problem, but others may benefit from this. The best soap cutter ia a taping knife, used by plasterers to coat the sheetrock tape on a wall. (these look like big putty spreaders with a straight blade and handle) These can range in width, but it is better to get one at least 10 inches, or preferably 12 inches wide. It is good to have the blade reach all the way across the soap you want to cut. Just hold the taping knife STRAIGHT UP AND DOWN and make one cut while holding your hands directly over the handle of the knife. It is also best to cut your soap as soon as it is hard enough to do so, preferably withing 48 hours, while it is still soft. Some of the soaps with beeswax and/or cocoa butter set up so hard it is impossible to cut them without chipping them. (Makes for a long lasting bar, though). They sell taping knives in any hardware or home improvement store, and at some discount stores. A soaping site is selling them for $12.99 each, but I got mine at a discount surplus store for $2.99. Some dollar stores even have them at times. Make sure you wash and dry it well before you put it away, as some of them tend to get rust spots on them. Not a disaster, but a nuisance to have to clean off the next time you want to cut soap. Good luck!