The outlet probably turns outward, so it’s not just vertical movement. You definitely need something flexible.
Maybe there was some kind of metal support that got left out during installation?
Foam seems unreliable. But maybe it could be injected from the crawlspace, not damaging anything in the bathroom.
The fiberglass is lighter, warmer and can take a lot of beating. If the enamel on a metal one gets damaged, it will rust quickly. Even if it doesn’t leak, it will need replacing because of aesthetics.
The outlet probably turns outward, so it’s not just vertical movement. You definitely need something flexible.
Maybe there was some kind of metal support that got left out during installation?
Foam seems unreliable. But maybe it could be injected from the crawlspace, not damaging anything in the bathroom.
The fiberglass is lighter, warmer and can take a lot of beating. If the enamel on a metal one gets damaged, it will rust quickly. Even if it doesn’t leak, it will need replacing because of aesthetics.