Does this scabbard really exist? My son has seen a kind of backpack scabbard in a Ninjago magazine, and would like to get a real one for his collection. It holds a single large sword - all the others he has hold two swords, either in parallel (in the style of the Witcher!) or in the form of a cross.

I have found this unofficial 3D model, but would rather be pointed towards the real thing if there is such a thing.
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