I used safety wire. Couple turns to tighten pigtail and tuck. Tucked down in nice and never had another issue.

 I removed the foam from the outside of the cracks (enough to expose the outer side of the cracks), epoxied the cracks and also filled the holes left from the removed foam and used a hose clamp to hold the plastic ring tight while the epoxy cured. Be sure to remove any epoxy squeeze out from the inside of the ring before the epoxy cures.

I wonder if a metal wire or strap like for cv boots would work to go around the outside and pull it all in tight? There's plenty of room on the outer side, right?

Super glue is not going to work. I made a metal band that fit tightly around the outside between the plastic and the foam

here it is

https://www.classiczcars.com/forums/topic/59887-horn-button-refurbish/?do=findComment&comment=547581