I use a cheese cutter I got on amazon and it works very well as a hobbyist. I just drew a line for a one inch bar on it in pen. It's this one: https://www.amazon.com/dp/B00004S1DU/ref=cm_sw_r_cp_apa_glt_fabc_RVAX9A6QBTMP11R9KAJ2?_encoding=UTF8&psc=1

I use this and it works pretty well. I had been using this but did not like it much. Wire cutters are really nice; the only downside I've found is it sometimes leaves bubbles (?) on the soaps, but that could just be user error.

Hey there 😎👀

So I just got this beautiful and functional cheese cutter off Amazon for $17. I like it because I could change the wire if I needed to. The cutter doesn't leave any texture on the soap after cutting. It's wood I just used permanent marker to draw some guides to cut my 1 inch bars of soap. The cutter works perfectly for my frosted tops my bars are like 3 inches wide and can be 6 inches tall. If your cutting soap taller then that I wouldn't recommend the cutter as the wire at the end are wrapped the wire so if your soap is too tall it will leave a semi circular cut out in the corner so I have a mark like 1 inch from the top of the cutter that I line the soap on so it doesn't leave that mark. I cut my bars on the side of the loaf so I don't drag my biodegradable glitter through the soap. Oh 👍 here is that link Soap cutter from Amazon