Just checked, My stud finder was not able to identify where the internal wood supports are in my FG roof. 2 out of 10 tries correctly identified the location, but the rest of the time it just began blinking an error message.
Rubbing alcohol works surprisingly well at cleaning rubber.
I've also found Syl-glide pure silicone sold at Napa in the grease section cleans rubber beautifully. Silicone spray works too.
Anything with Petroleum Distillates should not be used on rubber as it will dry it out and cause premature cracking. Unfortunately, most tire shine/ protection products contain significant amounts of PD's. Pretty funny eh?
Many months ago I bought Aerospace 303 which is promoted as a sunblock type of product safe for rubber/fiberglass but only recently applied it to my tires.
Not shiny at all, just made the tires black and look clean and mean. I took the wet applicator pad and used it on the black patches of my bumper which turned gray/ white from chrome polish contamination, and it turned them black again with so little effort I was amazed.
The VHB tape's bond will only be as good as the gel coat's bond to the fiberglass. If you can pull down the interior roof covering you will be able to see the wood ribs inside the roof. Once again, Stainless steel machine screws and large stainless washers on both sides with neoprene washers and a good adhesive sealant are the only way I'd feel safe putting a cargo carrier on the fiberglass roof.