Not sure what Jfox did but I've seen several guys remove a small piece of carpet where the hull is, glue wooden/plastic blocks to the hull and then use those blocks as a place to mount an amp panel.
Creating a panel for the amp, opposed to screwing it to the hull will save you time in the cubby and possibly save you from any issues. A local shop around here screwed an amp to that outside wall and sent a screw through the hull of an X2.
I used a panel that goes across the back of the cubby, supported by blocks that were screwed to the floor or the bow seat back. The rack on the right side I did actually screw to the fiberglass but I think I'd go the block route if done again.
http://i1092.photobucket.com/albums/...4/DSC_0163.jpg