Fighting handlebar vibration

Tl;Dr: Foam in handlebars is unlikely to help

I’ve been riding Jim’s Brompton. It goes fast, but the bars vibrate on gravel.

Adding foam to handlebars was an idea from motorcycling. Apparently – irrelevant to cycling:

https://www.reddit.com/r/gravelcycling/s/PvsXIk7Gxr