Wow mate that is such a dissapointing story ....
We learn lessons from such torment ,Id say yours would be that you will never ever get the same job that you could have done ,done better by someone else..
As I say mate I knocked mine up with 20 by 20 gal 1.6mm in a day ,Its strong enough once welded to support a bloody heavy load like right now mine is supporting 2 FC grilles about 6 bumpers ,2 elephants and a giant panda.
The gal is fairly easily fomed with your foot and a vice and also some larger diametre pipe to get the curve diametre your after.
One tip is to make sure you have the curve across the rear window right before bending your edge curves if that makes sense, It will once you get going.
Once youve got your overall shape you could cover it with plastic ,stainless ,alluminium or other car parts as in my case.
Although I made mine to sit on top of the tray edge it wont be hard to sink it in to the inside - my figuring was that the extra height would make the chopped roof look lower
Mine is covered with MDF and I used the reco'd tarp that came with the ute.
As I say mate - very dissapointing story and please please dont take it out on your wife again hehe