A few months ago, at the MakerFaire Brussels, my GF bought a “wonderful natural soap” (it's her words) from Alinessence and ask me a good box to protect her soap, even if she travel. The soap hasn't a standard size so I've made a fully custom box.

Really easy to print with a small 3D Printer but also to clean, this soap box can be used with many another soap brands.

If you need to add some custom modifications, I share the OpenSCAD files. Don't forget install the BOSL2 lib to modify source files.