There are a few holders for your katana out there, but I didn't really like any of them. Some of them use too much filament, some separate the blade and the saya too much, in others they are too close...

So I made this model:

• It is as minimal as I could made it, so the katana is the main focus

• I find the separation between blade and saya to be in the goldilocks zone, but you can easily modify the step file

• The screws are covered by the blade

• The holders themselves are a few mm wider so you can put some EVA foam and prevent the plastic from sratching the blade

• Used 6mm wall plug

Mirror the model in the slicer so the good surfaces are opposite to each other.