That mounting bracket is 4mm aluminium, but it's softer stuff than the traditional 6082 ali that's used. Should hopefully be robust enough though.

Can't honestly say how well 2mm ali will hold up, it'll all depend on how much force is going through your robot during each impact. But I used to mount my 900s with those mounting plates that come attached and they only broke after several large impacts, so you might be alright with 2mm ali.
Not got any more pics at the moment, but there's not much else to see from it's current state. More pics after the weekend probably, should be ready to rumble by that point