Probably more than one way to do this..... but..........
Strictly FWIW....
On our prior boat with an internal frp full head liner, I installed organizers to direct the lines back down to clutches on each side of the hatch.
I drilled thru with a small-size bit from the outside. Then used that small hole in the overhead to center a hole saw for a one inch hole (might have been 1 1/4", it's been a long time ago.)
Once the proper size holes -- for two or three axles -- were then drilled from the outside for the bolts, I had bolts that I had cut to length for going thru just emough to be flush with nylock nuts up against fender washers. I had to use some tape and temporary hold-onto's for the washers and nuts to get them started. One person to hold the outside while the other person tightened up the inside nuts. I recall losing one washer inside the head liner.....
It helped that there was about a half inch (+ or -) between the molded head liner and the bottom of the coring.
Once the new organizers were bedded and tightened, we painted some hardware store twenty-five cent "hole plugs" to match the color of the head liner and pressed them into place.
And, "Bob's your uncle" as they say.
You could see the pattern of those flat plugs on the inside but they looked "stock" and blended right in. (IMHO)
Loren