If you like to design your custom inset for the Modular Shower Soap Dish, download this templates which I made available under a free license. So you might even publish your custom design for the benefit of other users. The templates include a STL and STEP variant so that you can edit them for example with Tincercad or any other 3D application.

I added a sample print profile which you can use for your own inset design.

PLEASE DO NOT PRINT THIS SAMPLE PROFILE!

The sample print profile should serve as the basis for printing your own design of an inset for the "Elegant Soap Dish for your Shower". It doesn't make sense to print out the inset template itself because it doesn't have a water drain.

Simply remove the template object from this print profile and insert your self-created insert.