Never needed to use one, but...

I've a Jazz / Rockabilly hollow body archtop with 3 P90 pickups
that howls like a banshee anywhere near a guitar amp.
Theres's a guy in USA who sells made to measure F hole bungs to suppress feedback.
Which look like they are cut out and made from the soles of flip flops...

I used to use a Boss acoustic foot pedal with built in feedback suppressor - a notch filter of some sort..
But it was too much faff trying to use it on stage with a piezo pickup mandolin...