Oh I see. I think I misunderstood "in the rig" to be the solar panels in the rig. But you are asking how to mount fasteners to the fiberglass shell. I can give you some ideas, because it's similar to boats.
1) Maybe this is a bit "far out," but something like Sea Suckers with the ring tops you can tie to (these are ultra powerful suction cups). I have not used these myself, but many boaters do.
2) If you do want to put a mount point on the shell, then I would look for something that has some faying surface. In other words, some sort of base that you can get some bedding compound under. A simple eye bolt doesn't really have any place to put bedding compound. I'd also want stainless or chromed bronze (plain bronze is nicer on boats, but the Chinook isn't really a bronze type of look, IMO). So, I'd look to the boating world. Since any stainless should be plenty strong, 316 is better than 304 (more corrosion resistant). But either would do. Here are some ideas (I don't like things like rings that can flap around when not in use):
http://www.sea-dog.com/groups/1408-rece ... chor-point
http://www.sea-dog.com/groups/1423-eye- ... h-shoulder
http://www.sea-dog.com/groups/1442-diamond-eye-plate
http://www.sea-dog.com/groups/1443-oblong-eye-plate
http://www.sea-dog.com/groups/1256-tie-off-eye-plate
Sea Dog is a decent-stuff marine hardware company. They have a large selection. There are fancier things like Schaefer removable deck rings, but they are much more expensive (nothing wrong with that if it's what you want). Perko is another known brand, and there are others. Marine Deck Hardware is the general category you'd be looking for. Main thing is to make sure it will handle the load angle you will be placing on it (not that it's a huge amount, but the consequences of it coming loose could be fatal), and that you can bed it properly to avoid leaks. If you have a local West Marine store, you could lay hands on some of this stuff (albeit at a cost $$). I typically buy from Fisheries Supply (west coast), or Defender, Jamestown Distributors, or Hamilton Marine (east coast), if I'm not in a location with a local chandlery. If you need to buy fasteners separately, McMaster Carr has a decent selection of 316 stainless, oval head, and other things that can be the devil to find at the corner store (and they have fiberglass/GRP board).
As far as mounting, it's by far easier if you can access the back side. Of course the shower is there, but you do have access via the large locker hatch (and/or maybe behind the shower hatch, except I think that was moved to the side by your era?). The idea is that you want to put some sort of backing plate there, and a washer/nut. Not "just a pointy screw." (They don't hold all that well in fiberglass.) For a relatively light load, with loads in shear, a fender washer may be fine. Otherwise, a metal, fiberglass board (McMaster Carr is a good source), or plywood backing plate are other common choices. I'd stay away from cutting board or Starboard type plastic backing plates. It creeps under load (and doesn't take sealants well, not that that matters on the back). For something larger/heavier I'd go for a longer backing plate perhaps spanning both of the upper hardware pieces (vs. washers).
If there is already some coring or backing in the area, then your plan can be adjusted.
If you can't access the back side, there are other ways, but if you can, no need to type another "wall."

If you find that you cannot get to the back side, then we can noodle the other ways.
I like butyl tape (GOOD butyl tape, of which these days "Bed It" (sold by a sailor) is all I can vouch for) for a mechanically fastened item such as this. But other things could work too. If you are going to use a "caulk" type sealant, I'd go with 3M 4000 (polyether) because polyurethane will yellow in UV. Not that you are going to caulk around the outside of the backing plate (heaven forbid

), but there is often a tiny bit you can see peeking out around the edge anyway. 3M 4000 will "cure" and won't give you any ongoing ooze like butyl can.
BG