Wish I had a pic
I made wooden speaker boxes out of 1 x 6 boards. Three 6x6x1 pieces glued and screwed together (stained teak finish), with two of them having holes cut for the speakers. I then used a plastic mounting doo-hickey that used a thumbscrew clamp and attached them to the stern rail next to the stern rail seats. I ran the wires out of the locker hatches. Viola-no holes in the fiberglass. When not in use, I loosed the clamps, detached the speakers, and set them in the lockers to prevent theft or excessive weather damage.
I got the idea from someone that used teak winch pads in a similar fashion.
If I find a picture, I'll post it.