What size speaker is that? Can't tell if that's a thumb, index finger, or big toe,
Permatex black rtv would be less than $5 for a small tube and if you "gasket/glue" it right, it's gonna be the strongest part of the surround. Use like a split pea size drop on both sides, smash it in the crack (LOL) and wipe off the excess... You'd be hard pressed to even notice the repair.
I get that you probably feel you shouldn't have to but just saying, it's a lot cheaper than shipping the sub back,