The 4-Runner shell is more than just fiberglass, IIRC, there is a also a metal under the the fiberglass, at least it seemed so when I drilled through it to mount this rack I made for an '86 Runner I restored a few years ago.
I used two 1"x1" square stock to mount the rack to. The rails were bolted to the shell in 3 places using some 3x4" plate as backing material.
On my '90, I carry upwards of 100lbs of gear in it, and don't have an issue with the fiberglas shell either.
If you can't weld, I made my first one about 14yrs ago using 1/2" metal conduit, pre-bent 90's and fasteners. The supports were 2"x4" nail plates.
I have continued to use metal conduit, but now it is welded, and I bend the corners myself. This was the last one I made for my '91 flatbed.
