What's the generic name for this threaded/decorative "stem cap" on a Delta threadless steer tube extender I picked up one of these Delta threadless stem risers / steer tube extenders so I could try out different handle bars heights on my gravel bike without going through a bunch of stems (since shops near me aren't open for a proper bike fit).

I got it used and really cheap, but it's missing the plastic threaded &quot;stem cap&quot; (see picture). It's not a functional stem cap, per se, as the stem cap bolt actually threads through down inside by the clamp. I guess it's really more of a dust cover, and it threads into the top of the unit instead of being held in place by a bolt. The guy I got it from said it's a generic part that should be easy to replace. But I can't really figure out what it's called so I can order one online. The manufacture has not been helpful thus far.
There's a good chance I'll replace this setup with a proper riser stem once I figure out where I want my bars, so I'm more asking out of curiosity than an immediate need.
According to the installation instructions the manufacturer calls this a &quot;dust cover cap.&quot;  As you note, this is not a headset or stem cap since it doesn't mechanically engage with the stem to preload the bearings.
A Google image search for the image you included found a bunch of pages on Amazon describing this as a &quot;Delta Cycle stem raiser,&quot; and I found the instructions on the Delta Cycle page.
Unfortunately, it doesn't seem to be a generic part; I couldn't find a single instance of it being sold individually.  Perhaps you could bodge something with a normal headset cap.