Have you ever repaired/restored stripped threads on fiberglas cap? How did you do it, what did you use? Two-part epoxy maybe?
Like this one on roof as well as those on L-Shaped internal frame around windows.

 

I'd fill the hole with a two part epoxy like JB weld, then drill with the appropriate drill, and re-tap.

I would think that any hole that needed to withstand any substantial loading would have been originally fitted with some sort of metal insert

It would be possible to epoxy the hole closed then drill and install a metallic helicoil if repeated disassembly is anticipated.

 

A helicoil has the same problem; it's entire pull-out strength is just threads into FRP (Fiberglass-Reinforced Plastic).

One option is a TeeNut https://en.wikipedia.org/wiki/T-nut . They're designed for wood so the prongs will probably not anchor well enough, but a little epoxy could leave you with a very well reinforced threaded steel "hole." An advantage of a TeeNut is alignment. Drilling a filled-hole (for threading) in exactly the right place might be difficult. With the TeeNut, you can thread the TeeNut onto the bolt, and with everything assembled tighten it up to pull the prongs into the base material. Add some epoxy, and it will be well-aligned.

 

Not necessarily true, if the helicoil is appropriately cemented into the base material. The surface area footprint of the helicoil is substantially larger than the original threading into the fiberglas, as well.

It is my experience that most fiberglas hole failures result from repeated disassembly and reassembly over the years,

Any metallic insert obviates this repetitive wear.

 

Thanks a lot, guys. These holes are just for the trim. Above is for edges of the headliner, while the rest are for the plastic L-trim, so no significant loads. Screws are self tapping. I'll try JB Weld. If to ohard to self-tap I'll try 2-part epoxy.
Regarding reinforcement, the clearance holes for mounting the cap on cabin and bed seem to have washers molded in. I believe the panic handle mounts also have the inserts molded in.

 

I filled my with fiberglass resin and that worked well. I had first tried filling them with hot glue and that didn't work at all. The helicoil idea has merit, and if the trim panels come off a lot I could see that being a good way to go.

 

The J&B weld epoxy is amazing stuff because it wets very well as it cures. If you can find a knurled brass thread insert I would open up the hole and bond the insert into the hole with J&B Weld. I doubt you will ever have another issue with the assembly after that.

Many years ago I worked for Wind Power Systems. We had a manufacturing procedure to bond 3/8-16 UNC brass thread inserts into the ends of the windmills blades using epoxy that was very similar to J&B Weld. The 1/8" thick aluminum blade tip plate was secured with a pair of 3/8-16 UNC button head screws and had balancing weights inside the tip plate with a pair of steel clam shell weights that preceded the leading edge of the blades profile to stop blade flutter. So the blade tip assembly was quite heavy and went hurling around at about 120-140 RPM when the windmill was running and making power. The blades were 12 meters long so the angular velocity was quite high at the blades end. The only time this fastening system would fail was when the blade would strike the tower leg when an overspeed-shutdown went haywire. You made yourself scarce when a machine went into overspeed and started its shutdown process because if a blade strike occurred the tip would come off the blade at anywhere from 6 to 7 o' clock and it was moving. On one such occasion the chief company engineer wanted to see what was happening when a forced overspeed was initiated and the blade struck the tower and the puff in the sand from the tips trajectory was easily 150 yards away from the tower and I never saw it until the sand kicked up and I was able to retrieve it. The brass inserts were with the machine screw and the epoxy and some fiberglass from the blade was present as well. Later the blades were replaced and the damaged blade from which the tip assembly had come from was like a TNT charge had been placed in the end of the blade. It was completely obliterated for almost 6 inches and indicated that it had been quite an impact.