BoxRec

I can't think of any at the moment but would be very interested to find out. Only one that comes to mind is songwriter Anar Badalov who I think has had some sort of amateur career...

Ernest Hemingway
Jim Tully

Hemingway's obsession with boxing is not the same as being a boxer. Far as I know he was never an amateur, just did a lot of spars and matches in his home against friends. I recall a story where he claimed he beat some professional boxing friend of his that he sparred with, and the friend took exception to this and soundly knocked the snot out of Hemingway showing he took it easy on him anytime they ever sparred previously.

Songwriter (and singer) Paul Thorn, who went 10-3-1 as a pro and fought Roberto Duran.

Robert E. Howard may have had some amateur fights, though I’m not sure if they were sanctioned events, sparring matches with friends, or took place in his mind. At any rate, he was a big fan of boxing.