Would be really interested to see if someone posts here and what they know/suggest. I've always wanted to that as well but own a maloo now and not willing to do that to it.
Haven't researched it but have given a fair bit of thought.
Big job to do it properly . I'm assuming you'd want to take it down vertically from the back window. Means you mite need extra forming in the new wall since it's now vertical instead of angled.
It means you'd need a new custom fuel tank and cavity. Shorten up the bolt down side and floor panels. Bring tray drains aft to the new back of the tray .
Mite need strengthening down at floor on sides if you take new wall all the way to floor.
If you are seriously thinking of doing this extent I'd be talking to an engineer as it should be engineered and they'd be telling exactly what they want.
If you just wanted a little bit of extra storage to fit small to medium sized things then a much simpler idea would be to cut a hole in back wall, make a hinged or sliding door, sliding could be bigger.
Then make removable bolt in box, install in tray, bolt and seal to back wall.