The best way to install some mounting in the ceiling or anywhere else on the shell would be to screw and glue a hardwood block directly to the fiberglass.
In my case I have Ensolite insulation, but the idea is to cut the insulation and inside covering out carefully to match the shape of the block you want to attach.
Carefully shape the mating side of the block to fit the shell.
When you are done with that drill a few small holes spaced to support the block.
When done with that carefully countersink outside of the holes to match some stainless flathead screws to screw into the block.
Have someone hold the block in position while you drill some pilot holes using the previous holes as guides.
Clean off all of the dust and put a little glue on the block and have your helper hold it again and then put a dollop of glue in the hole from the outside and install the screws.
By the way this is the way that the rear cabinets are installed and I have never seen a leak or loose screw in those.
I also mounted all of my cabinets and bulkheads that way using screws and polyurethane adhesive and no leaks
, no loose screws over 30 K miles and 6 years.
Here is an example of how I mounted the overhead cabinets in my Scamp