The dust cap has no sound-generating function whatsoever. It's there to keep dust from getting into the voice-coil and magnetic gap--the motor of the driver. I don't know what adhesive is being used but none of my dust caps have ever fallen off of any speakers of the many brands I've owned. A neighbor's cat once dented the dustcap of a KEF driver in a transmission line sub I'd built, blah, blah.
Anyway, call Brent or JC re the cap. I think Super Glue would be fine but check with them. Of course, if it isn't very dusty up there in Grand Rapids, you could leave it off. It might look peculiar, however.
Axiom Resident Expert