I'm running fiberglass rocker panel covers. I like them, but I've had a problem with the mounting holes on the top breaking out at both the front and rear mounts in the fender wells. This is my 2nd attempt to fix the passenger side.

I realized that there wasn't a lot of meat left after the hole was drilled to keep the cover from vibrating and breaking the mounting hole. My previous attempt was to use fiberglass only to rebuild the area. This time around I cut thin strips of sheet metal to serve as fiberglass "rebar" on the outer section of the cover.

I opened up the existing hole and sanded the top down. Next, I glued the "rebar" in place before using fiberglass cloth & resin on both sides of the area. Repainted the whole thing semi-gloss black. Installed it, marked where the holes needed to be and drilled fresh holes. Finally, I picked up some stainless steel screws and nut plates to attach the front & rear.

It feels plenty strong and I'm pleased with how it turned out.

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Good job.

You could also 'glass on a piece on sheetmetal on the back side. Bend it so it follows the shape of the panel and then drill your hole through it.

Looks Good !!
I'm working on my rockers at this time also. But they are the originals.

That’s what I will do with my Fiberglass GT valence.

Worked in the sticky s#it for years ,no issue incasing sheet metal brackets in the there,just make sure you "rough them up" with a very course grit so they bond well .
You can fit treaded or captive nuts to brackets just fill the holes with modelers clay or similar before glassing on.

I like this idea better than my solution. If they give me problems again, I'll go this route.

That's what I did with mine (thin backside reinforcement) and simply bonded it with JB Weld. Been there for many years DD without any issue.