10mm ali would require no welding what-so-ever, but Im sure 2mm steel - welded - would work (though Id imagine 2mm steel to be quite vunerable to deformation) maybe a sub-chassis of steel box section with 2mm steel panels bolted on would be a good idea (though more costly).

The triangles basically work like this on Biohazard (american robot) - http://www.teamkiss.com/Vegas/Biohazard.jpghttp://www.teamkiss.com/Vegas/Biohazard.jpg
they prevent any edges from being exposed to, say, horizontal spinners. In biohazards case, this is just done with hinged skirts cut off at particular angles, though this would be easy to implement on your design too.