My wife recently bought the Ninja Blast blender, and wanted to keep the blades covered while the other parts are being washed or drying. This is a cap that uses the locking mechanism to stay in place.

There's a notch for the printing seam and to align with the arrow on the Ninja Blast. Simply push the cap in when the notch/line is a bit over 45 degrees to the right of the arrow then twist to lock after the cap has been fully pushed in.

It should be a fairly snug fit so if there is any play after twisting the cap in, I recommend very slightly increasing the size of the print.

STEP file is there for further edits.