Very interesting discovery! I knew the roof was fiberglass. But had no idea on the back wall. My cab is slightly shorter than yours so my guess the side portions are more aluminium at mine.
Working with fiberglass isn't anything extremely exotic. First option in your case would be attaching a piece of aluminium sheet from the inside of the fiberglass wall to cover that window opening. I'd fit it with blind rivets putting their heads slightly deep into the surface so you could cover them with filler later. Than take a big can of glass-filled epoxy filler and wipe 2-3 layers of it onto the alu. After that you need an agressive orbital sander to smooth up the surface and than later a standard pre-paint procedures of sanding-primering-sanding.
That described way has one big negative point though. Aluminium has high thermal expand coefficient. So you'd get risk of local deformation of that alu base when the truck is exposed to bright sun with filled area cracking. Alternative options could be using a kind of net or grill (stainless steel, alu, fiberglass, fabric) instead of the metal sheet. You'd need a kind of support from underneath to apply filler. I'd take a sheet of alu or plywood or so again but just put it temporary at the back. And insulate with a piece of polyethelene film (or so) for the filler to not glue up to it.
If you're able to position the cab the way its back wall occures horizontal you may use epoxy (or polyether) resin instead of filler. Some kind of net anyway or you may use glass fabric or glass mat (as it's called here). Resin penetrates such stuff better so the mesh of threads could be tighter. You may apply 2-3 layers of glass fabric over each other penetrating with resin so you'd get actual fiberglass. Important point the edges of the existing hole must be grounded and sharpened for your new epoxy material to make overlap on the existing sheet, as more as better.
Ok, one more interesting option is if you may find a sheet of ready flat fiberglass. A portion of a board of a boat or IDK what. This way you use it instead of aluminium plate not suffering thermal deformation effects.
Well, one more idea. You may find a stock rear window off a R-model cab (with rubber), cut your cab for it and go Australian style!