Unless you have access to laser, water or plasma, cnc type cutting machinery,
I'd give the "diy" a miss on this project. I suppose you could maybe do it with a band or coping saw type thing but your best bet would be to dodge the hassle and go for one the following:
http://www.songofthepaddle.co.uk/forum/showthread.php?13653-Yukon-amp-Nomad-Fireboxes I've got one of these, had it for a few years now. I
believe it's just a nimblewill anyway, made with the benefit of access to industrial cutters. The plates etc are perfectly cut, as you'd expect with a cnc machine but the bit's that
aren't made on a machine are frankly a bit pathetic, being simply round stainless bar for the pot rests and bent stainless wire for the securing pins, with very little thought put into how they actually work. The pot rests tend to slip off the sides of the box in use and the securing "pins" are far too loose a fit. I made a set of pot rests from stainless hexagonal rod, with 90* bends on the ends to stop them sliding around and replaced the supplied bits of bent wire with stainless split pins. It's a good piece of kit but only if you add your own modified bits.
Then there's this version:
http://www.songofthepaddle.co.uk/forum/showthread.php?35243-New-Firebox If I were going to be buying a new flat pack stove, it'd be this one I'd go for this time around. I've no idea how good this one actually is, it just
looks a better bet than the Yukon.
I wouldn't go for anything less than 1mm thick in stainless, I've got a firebox made by a member on here which is excellent in use but a lot thinner and it's quite badly warped, particularly the hearth plate. It's not possible to dismantle this one now, though that's not an issue as I only use it in the garden or if I'm in a vehicle with plenty of storage space. Regardless of that though, my Yukon is apx 1.5mm stainless and that's quite warped too. To be honest, it goes with the territory, repeated heating, cooling, occasional quenching etc will lead to almost any woodburner warping, I'd imagine even titanium would probably bend and flex with the heat, though I've no experience of that.
Alloy, going off my experience, will melt if you get the same sort of heat I get with my Yukon. It regularly glows to orange heat and You'd need to go to something like 2mm (or more) in alloy to have much chance of avoiding melting.