My roof is pretty much the same as your roof. I mounted my solar panel with DIY aluminum brackets made out of 1.5" aluminum angle "iron" and 3m VHB tape. I cleaned my fiberglass with acetone and roughened it with fine sandpaper before mounting. So far I've only mounted one panel but plan on two. The mounts are very solidly adhered to my top. The deflection you feel isn't a problem for such a light load spread over four 8" x 1.5" brackets (total = 48 sq in) You can see my installation here:
https://www.sportsmobileforum.com/fo...p-26528-3.html
You can pretty much ignore pages 1-2, although you can see a photo of my roof on pg 1.
Note that my brackets are at a slight angle from horizontal to accommodate the shape of the roof. I determined the necessary angles and mounted them to the panel before using the VHB tape. You can see that the outboard hole in each bracket is elongated to allow adjusting to the right angle. The tape isn't repositionable and be careful not to contaminate the sticky surfaces with dust, fingerprints, etc.
I attached the brackets to the panel, applied the tape and used two 3/4" thick sticks to keep the brackets off the roof until I was sure of the positioning. I then lowered the panel into place by removing one stick at a time. It's important to make sure the brackets are firmly pressed down to ensure adhesion.
A. I purchased Renogy's 100w panel that is only 20" wide and fits well on my roof's central "hump", with very little overhang at the edges:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
B. Flexible, adhesive backed panels as suggested by Lilnuts2 will be significantly more expensive and might not fit well on your roof. Although better than earlier flexible panels, they're still more susceptible to heat build up. You can't use your existing metal structure for mounting since it's not tight against your roof.
C. 3m VHB tape is extremely strong (tensile strength is about 100 lbs/sq in) and better than penetrating your roof. If, for some reason, you prefer to screw mount your panels, metal backing plates the same size as your brackets should provide enough strength. Should you ever want to remove your panel(s), there are instructions online for removing VHB tape.
https://www.3m.com/3M/en_US/vhb-tape...ransportation/
This is what I used:
https://www.3m.com/3M/en_US/vhb-tape...ransportation/
If you have any questions, feel free to send me a private message.
And thanks for your photo of the metal framework. I haven't removed my headliner so your photo helps me see what it's like. Too bad it's not flush against the top so we'd have the additional headroom.