Does this scabbard really exist?
It seems to me drawn item might not represent existing element accurately. It is very close to Minifigure Armor Shoulder Pad Single with Scabbard for 2 Katanas. Could be pictured at an odd angle.

OR
Some modification of Minifigure Armor Shoulder Pads with Scabbard for Two Katanas and Bar Hole

The only one I could find for holding single katana is Minifigure Armor Shoulder Pad Single with Star Lettering and Scabbard for 1 Katana, but shoulder piece is covering minifigure's right side.

There are some other elements that could hold two katanas:
Minifigure Armor Shoulder Pads with Scabbard for Two Katanas

Minifigure Scabbard for Two Katanas