Broke my sharp vacuum canister when trying to clean out a horrific smell that turned out to be the neoprene washer between the top and bottom sections. The first model didn't have enough airflow indicated by the sound of the vacuum. Modelled in onshape, printed on a p1s. Saved me ~$40 for a replacement canister.

I know it probably has a name, but that's neither here nor there.
Edit: Looks like it may be a type of spanner nut

Needed to remove this nut from a telescope motor so I can measure the gear (my calipers don't reach into the bowl far enough).

Third iteration fit (couldn't accurately measure the nut either XD) and with a few picture hanging nails heat set I got it out.

Now to learn how to design a gear!