It's the same fit as  https://www.printables.com/model/302219-customisable-stamp-seal-stamp.

There you can find a blank stamp to add your own hanko seal.

The base model has a ~21mm size and the seal itself shoud be the size you want, matbe is possible to exceed the size of the base stamp, but i haven't tried.

THe idea os the Hanko Stamp comes from https://www.printables.com/model/523995-japanese-inkan-yin-jian-aka-hanko-pan-zi

Mine is almost a copy, but I didn't use their files.

i think it should no be used as a real hanko. Maybe takarocks design.

I have printed slot up and managed without supports.