Well here's my progress today. Sounds like I've done pretty similar to you with that piece of steel, . The 'corner tabs' will no doubt make getting the box on a bit more difficult, but I thought they were worth a try, if all else fails I'll cut them off. I'm thinking that I'll try having the 'floor' of the box sloping down & then levelling out to make it deeper. I think it'll look better with a slope than with a sunken box in the middle of it like i've seen on a diy elsewhere, & should give a little more space. I'm also thinking that I may not put a top on the box because the extra height would be useful so I can discreetly leave the iPad in there charging, & the only way i could fit the iPad in would be without a top. Still haven't decide exactly how I'll go yet, may have to make a two part box & fix it together once inside the space. Still playing with cardboard mock ups.
The bit that I'm most worried about is the hinge. There seems very little to attach it to, but knowing that you & others have successfully managed it gives me confidence that I'll manage.
My hole looks pretty much like your hole (which is hardly surprising) except for the two bottom corners.
The piece of steel with the section I cut out of it.
The piece of steel fitted back in, & rivetted to the plastic 'corner tabs, with the original glovebox replaced. Getting the almost hidden screw at the right hand end of the piece of steel screwed back in was achieved with a torch, a mirror & a piece of blu tack holding the screw on the end of the screwdriver & patience.
The four screws holding the original glovebox that can be seen will screw into the 'box', holding it securely at the front ....I think.
Now to make a shopping list for Bunnings.
Cuppa