What is the structural component of this vertical fin?
If the structure is the foam then you can just get some epoxy in there and it will be strong as new. If it is foam with a fiberglass covering then just butt gluing it back together is not going to be strong enough.
You can heat most epoxies to make then thinner, heat will also dramatically reduce pot life so be ready, your 30 minute epoxy once warmed up may only give you 5 minutes.