It's an unstressed joint -- good caulk _should_ be enough, IMHO. [Others may disagree.]
Use sheet-metal screws, short enough so they don't extend into the cabin.
Drill the holes in the outer fiberglass just a bit bigger than the screw shaft diameter. If the holes are too small, the screw threads will crack the glass when you drive the screws in.
Use masking tape around the edge of the big hole to control "caulk mess".
Put a bead of caulk on the speaker, a blob on each screw hole, a thin layer on each screw. Tighten the screws enough to get a bead of caulk extruding around the speaker -- don't try to drive them all the way in.
The "authorized" method is to remove some of the core around the screw holes. Then, fill with thickened epoxy. Let it set. Drill out the screw holes.
Then proceed with caulk as above.
I'm not willing to put screws into "wet" expoxy -- too frightened they'll never come out.