I have a #10 that fell out because the acorn more or less rotted off.

Here’s a picture of it next to a #8 that has the right external dimension but
The wrong nut inside it. The shell is 7/16 hex, the nut inside is 3/8 hex so
Some of what’s holding it together is Forming on the edge of the “washer”.

Maybe I’m not feeling creative enough at the moment but not sure how I’d duplicate that
Without investing in tooling.