That image was my attempt at a quick mockup, and an upload to ausubaru. I think I failed ad both.
As said earlier there are plenty of opinions as to what it would take to give a car it's rigidity back after you have taken a tin opener to it's roof. There is an opinion that welding up it's rear doors will stop it's back sagging like a daschund. That could be true. Personally, without any engineering background, I would have liked to see some diagonal reinforcement from the top of the B pillar to the rear wheel well. Not a roll cage, but body work. Or a roll cage covered in body panels. That would give it that sloped side profile of the old Brumby, plus add rigidity.
One also have to think about the bin pinching in from the sides. Adding a rear window would counter that to some extent, but how can one stop the tailgate opening from collapsing over time. The only way I can think of is adding ribs from the one side to the other, under the floor of the tub. Looking at some examples on youtube, this usually involves 25mm square box section. Looks horrible but I suppose it is acceptable?
The most meticulous looking conversion that I found was some Scandinavian bloke cutting up his Volvo. You can tell he knows how to wields that grinder and reciprocating saw he serious about it. Not just hacking away at it as most youtube efforts. But as good as it looks on the outer skin, I do not agree at all with the style of the bin. Very small, and made out of aluminium tread plate.
https://www.youtube.com/watch?v=EOxiDJ1M8_A&t=25s
I recon take his approach, but do a proper custom sheet metal bin, and rear window panel. It would take a lot of skill with sheet metal working and welding, but it is perhaps a nice place to learn the art of shrinking, stretching and welding sheet metal. In the back of a ute, under a thick layer of rubberizing. That would hide most of the learning curve.