This threaded cap mechanism provides an M24 opening through which the soap tank can be refilled.

I tested it in PETG with 0.2mm layers, but there really should be no issues with other materials as well.

Use an xacto knife or the like to cut a hole in the top of the tank. Then fit the printed cover, using some glue if necessary. After filling up with soap, a coin can be used to turn the cap until the tank is fully closed.

FreeCAD source file (currently 0.19.3) is provided in case customization is needed.